Scanning electron microscope

ABSTRACT

A stage on which a sample is placed is driven through feed screws rotated by pulse motors which are controlled by a micro-step drive control method. Backlash quantities and feed screw pitch errors have previously been obtained and stored in a memory, and when the stage is to be driven, a stage controller corrects the backlash and pitch errors.

This is a Divisional application of Ser. No. 08/540,032 filed Oct. 6,1995 now U.S. Pat. 5,646,403.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a scanning electron microscope used toobserve fine structures such as those of semiconductor devices. Moreparticularly, the present invention relates to a scanning electronmicroscope having a structure which makes it convenient for an operatorto control an observation field which is set on a sample.

2. Description of the Related Art

In a conventional scanning electron microscope, an electron beam isemitted from an electron gun, converged by a converging lens anddeflected by deflection coils. The deflected electron beam is passedthrough an objective lens to scan the surface of a sample in atwo-dimensional manner.

Then, secondary electrons emitted from the sample surface are convertedinto an image signal through a secondary electron detector or othersimilar detector, and a magnified image of the sample is displayed onthe screen of an image display device (e.g., a CRT display) on the basisof the image signal.

The observation magnification can be set to a desired value within apredetermined range by the operator, and the electron beam scanningrange, scanning speed, etc. on the sample are adjusted in accordancewith the set observation magnification. A range that is specified as anobservation object by the operator within the electron beam scanningrange on the sample is an "observation field", and an image of thesample in the observation field which is magnified at the setobservation magnification is displayed on the screen of the imagedisplay device.

There are two methods for moving the observation field on the sample tothereby move the observation image of the sample on the screen of theimage display device: a mechanical field moving method in which a stageon which a sample is placed is moved by a movement controller; and anelectric field moving method in which the electron beam scanning rangeon the sample is moved by an electro-optical system control circuit, oran image data selecting region in the scanning range is moved, therebymoving the observation field.

The electric field moving method has the advantage that the image can bemoved at a constant speed when the observation magnification is high. Onthe other hand, the electric field moving method suffers from thedisadvantage that the oscillation width of the electron beam is limitedto a value which is much smaller than the amount of movement of thestage. The mechanical field moving method, in which the stage ismechanically moved, has the advantage that the observation field can becontinuously moved, for example, to a region on the sample which is awayfrom a region which is presently scanned with the electron beam.Therefore, the operator generally uses both the mechanical field movingmethod and the electric field moving method by changing from one to theother according to need. A device which enables the two methods to beautomatically changed according to the observation magnification hasalso heretofore been used. In this regard, as a conventional stagedriving system for mechanically moving the observation field, a feedscrew system has heretofore been used in which the stage is fed bydriving a feed screw with a pulse motor (i.e. stepping motor) which isdriven by a full-step drive control method (or by a half-step drivecontrol method).

As a mechanical field moving method, a position control method hasheretofore mainly been used in which a distance by which the observationfield is to be moved (hereinafter referred to as "moving distance of theobservation field") is calculated according to the amount of rotation ofa track ball, and the observation field is moved on the sample by thecalculated moving distance in a direction corresponding to the directionof rotation of the track ball. The track ball is a tool in which arotating member that is buried in an operator console is rotatedtwo-dimensionally, thereby causing pulse signals to be generated fromtwo rotary encoders.

Another device that is used in the mechanical field moving method is ajoy stick. The joy stick is a tool in which a shaft for control isrotated two-dimensionally to change output values of two potentiometers,thereby setting a direction of movement and a speed of movement in atwo-dimensional plane.

The above-described conventional techniques suffer, however, from thefollowing problems: Since there is backlash in a feed screw that isdriven to move the stage, when the moving direction of the stage isreversed, such a phenomenon is likely to occur that, although the pulsemotor rotates, the stage will not move for a short while, or that thestage slightly moves in a direction reverse to the direction in which itshould move.

The above-described undesired phenomenon is particularly remarkable whenthe operator moves the stage by open-loop control while viewing thesample observation image displayed on the image display device. Thus, asense of incongruity is caused, and it is difficult for an operator tocontrol the stage accurately.

Further, to rotate the observation field on the sample, the conventionalpractice is to tilt the electron beam scanning direction. However, ifthe stage is moved in order to observe a region which is adjacent to theobservation field in a state where the image displayed on the imagedisplay device has been rotated, since the moving direction of the stageis determined on the basis of a coordinate system used before therotation of the observation field, the sample observation image moves ina diagonal direction and is therefore difficult to observe. For example,if the observation field is controlled so as to move toward the right orleft side, with the observation field rotated clockwise through 45° fromthe X-axis, since the stage moves in the direction X, the sampleobservation image displayed on the image display device moves in adirection which is counterclockwise tilted at 45° with respect to theX-axis.

The conventional scanning electron microscope further involves theproblem that, when the operator moves the observation field by themechanical field moving method, that is, by mechanically driving thestage, while viewing the sample observation image displayed on the imagedisplay device, since the sample observation image moving speed varieswith the observation magnification, a sense of incongruity results whenmoving the sample observation image, and it is difficult to place theobservation field in a desired position. Particularly, when theobservation magnification is high, the sample observation image moves athigh speed, and therefore, it is extremely difficult to place theobservation field in a desired position.

Further, the conventional scanning electron microscope suffers from theproblem that, since a feed screw (e.g. ball screw), which is driven tomove the stage, involves a pitch error, it is difficult to put theobservation field in a desired position accurately by open-loop control.That is, even if the pitch error of the feed screw falls within themanufacturers' specifications, the positioning accuracy required for thestage may be more stringent than the specifications. In such a case, theabove-described problem arises. Particularly, when the observationmagnification is high, the effective observation field range, whichcorresponds to the effective display area of the image display device,becomes extremely narrow, and the positioning accuracy required for thestage becomes extremely high. Thus, the feed screw pitch error must betaken into account.

In view of the above-described circumstances, a first object of thepresent invention is to provide a scanning electron microscope whichenables an observation field on a sample to be moved to a desiredposition by a mechanical field moving method even if there is backlashin a stage for positioning the sample.

A second object of the present invention is to provide a scanningelectron microscope in which, when an observation field on a sample isto be moved toward a neighboring region by a mechanical field movingmethod in a state where the observation field has been electricallyrotated, it is possible to move the observation field independently ofthe angle of rotation of it while viewing a sample observation imagedisplayed on an image display device, and in which positioning of theobservation field is easy.

A third object of the present invention is to provide a scanningelectron microscope in which, when an observation field on a sample isto be moved by a mechanical field moving method on the basis of a sampleobservation image displayed on an image display device, the sampleobservation image can be moved at a constant speed on the image displaydevice independently of the sample observation magnification no matterhow high it is.

A fourth object of the present invention is to provide a scanningelectron microscope in which, even if there is a pitch error in a feedscrew of a stage for positioning a sample, an observation field on thesample can be accurately moved to a desired position by a mechanicalfield moving method.

Meanwhile, if a method in which the sample is manually moved through thestage is used, the observation field can be readily moved in a widerrange than in the case of a method in which the observation field iselectrically changed over. However, the manual stage moving methodsuffers from the problem that a great deal of time is required to searchfor a desired image because the image continuously moves. Further, whenit is desired to display an image which is adjacent to the presentlydisplayed image on the screen, for example, it is difficult to make thestage come to rest accurately at a point of time when the neighboringimage is displayed on the screen. Therefore, the operator is likely topass the desired image or lose sight of it.

Particularly, when the observation magnification of the scanningelectron microscope is high, since the image moves at high speed inresponse to the drive of the stage, it is extremely difficult to allowthe observation field to come to rest at the desired position.

In view of the above-described circumstances, a fifth object of thepresent invention is to provide a scanning electron microscope in which,when the observation field is to be moved by moving the sample, theobservation field can be moved to a region on the sample which isadjacent to the present observation field at high speed and with highaccuracy even if the observation magnification is high.

Regarding the above-described conventional techniques, when a sample asan object of observation is a regularly arranged device such as asemiconductor memory, it is sometimes desired to fix the movingdirection of the observation field on the sample in a predetermineddirection. However, when a joy stick or a track ball is used to input anobservation field moving direction, since it is difficult to accuratelyrotate the control shaft or the rotating member only in a predetermineddirection, it is difficult to move the observation field only in apredetermined direction by manual operation.

When the moving distance of the observation field is short, the amountof rotation required for the rotating member of the track ball isrelatively small. Therefore, controllability is good. However, when aposition on the sample which it is desired to observe is distant fromthe present observation field, and hence the moving distance of theobservation field is long, the amount of rotation of the track ball mustbe increased, causing controllability to be degraded.

Particularly, in the case of the track ball, the rotational angle of therotating member is not limited, unlike the joy stick, and the rotatingmember is isotropic. Therefore, it is extremely difficult to accuratelyrotate the rotating member only in a predetermined direction.

In view of the above-described circumstances, a further object of thepresent invention is to provide a scanning electron microscope in whichan observation field on a sample can be accurately moved in apredetermined direction even when an input device such as a track ballis used (a sixth object of the present invention), and in which theobservation field on the sample can be readily moved to either a nearposition or a distant position (a seventh object of the presentinvention).

SUMMARY OF THE INVENTION

First to fourth scanning electron microscopes of the present inventionfor attaining the above-described first to fourth objects are common toeach other in that a surface of a sample is scanned with an electronbeam, and an image in a predetermined observation field on the sample isdisplayed on an image display device by using an image signal obtainedby detecting secondary electrons emitted from the sample, and in thatthe scanning electron microscopes have a feed screw-driven stage fortwo-dimensionally moving the sample on the electron beam scanning plane,two pulse motors for rotationally driving two feed screws, respectively,of the stage, and a micro-step drive controller for driving the twopulse motors by a micro-step drive control method.

The first scanning electron microscope for attaining the first object ofthe present invention has a backlash memory for storing an amount ofbacklash observed when the moving direction of the stage is reversed,and a field movement control unit which, when the observation field isto be moved by a predetermined amount on the sample, corrects thepredetermined amount of movement on the basis of the stage movingdirection immediately before the present time, a st age moving directionto be taken subsequently, and the storage contents of the backlashmemory, and which drives the pulse motors through an angle correspondingto the corrected amount of movement through the micro-step drivecontroller.

The second scanning electron microscope for attaining the second objectof the present invention has an image rotating unit for rotating animage displayed on the image display device through a predeterminedangle, and a field movement control unit which drives the pulse motorsthrough the micro-step drive controller so that, when the observationfield is to be moved on the sample, the stage advances in a directiondefined by the angle through which the displayed image has been rotated.

The third scanning electron microscope for attaining the third object ofthe present invention has a magnification changing unit for changing theobservation magnification of the image displayed on the image displaydevice, and a field movement control unit which, when the observationfield is to be moved on the sample, drives the pulse motors through themicro-step drive controller so that the stage advances at a speedobtained by multiplying the moving speed before the observationmagnification change made through the magnification changing unit by theratio of the observation magnification before the magnification changeto the observation magnification after the magnification change.

The fourth scanning electron microscope for attaining the fourth objectof the present invention has a pitch error memory for storing pitcherrors of the two feed screws of the stage, and a field movement controlunit which, when the observation field is to be moved by a predeterminedamount on the sample, corrects the predetermined amount of movement onthe basis of the pitch errors stored in the pitch error memory, andwhich drives the pulse motors through an angle corresponding to thecorrected amount of movement through the micro-step drive controller.

In addition, the present invention provides a fifth scanning electronmicroscope for attaining the above-described fifth object. The fifthscanning electron microscope has: an electron beam scanning device forscanning a surface of a sample with a converged electron beam; asecondary electron detector for detecting secondary electrons emittedfrom the sample and for converting them into an image signal; amagnification and field setting unit for setting a sample observationmagnification, and an observation field on the sample; an observationcontrol unit for controlling the condition of electron beam scanningconducted by the electron beam scanning device on the basis of theobservation magnification and observation field set through themagnification and field setting unit, and for selecting an image signalcorresponding to the observation field from the image signal; and animage display device for displaying an image in the observation field onthe sample by using the image signal selected through the observationcontrol unit. The fifth scanning electron microscope further has: asample positioning device for two-dimensionally moving the sample on theelectron beam scanning plane; a field moving direction designating unitfor setting a direction of movement of the observation field; and afield movement control unit for moving the sample positioning device bya distance equal to the width of the observation field in a movingdirection set through the field moving direction designating unit.

In this case, one example of the sample positioning device has a stagecapable of two-dimensionally moving with the sample placed thereon, twofeed screws connected to the stage, pulse motors for rotationallydriving the two feed screws, respectively, and a micro-step drivecontroller for driving the two pulse motors by a micro-step drivecontrol method.

One example of the observation field that is set through themagnification and field setting unit is a region variable in shape,which is selected as desired from the area on the sample within whichthe sample surface is scanned with the electron beam.

In addition, the present invention provides a sixth scanning electronmicroscope for attaining the above-described sixth object, in which asurface of a sample is scanned with an electron beam, and an image in apredetermined observation field on the sample is displayed on an imagedisplay device by using an image signal obtained by detecting secondaryelectrons emitted from the sample. The sixth scanning electronmicroscope has: a sample stage for two-dimensionally moving the sampleon the electron beam scanning plane; a moving direction input device forsetting a two-dimensional moving direction of the sample stage throughencoders for two axes each of which converts an amount of displacementinto an electric signal; a moving direction limiting unit for settingthe sample stage moving direction to any of three directions, that is, apredetermined first direction (direction X), a second direction(direction Y) intersecting the first direction, and a two-dimensionaldirection; and a stage controller for driving the sample stage in acomponent direction selected through the moving direction limiting unitwith respect to a moving direction set through the moving directioninput device.

In this case, it is desired for the moving direction input device to seta two-dimensional moving speed of the stage in the first and seconddirections, and the stage controller preferably drives the sample stagewith a speed component in a direction selected through the movingdirection limiting unit of a moving speed set through the movingdirection input device.

One example of the moving direction input device is a track ball havinga rotating member, and two rotary encoders for converting the rotationalangles of different surfaces of the rotating member into electricsignals.

In addition, the present invention provides a seventh scanning electronmicroscope for attaining the above-described seventh object, in which asurface of a sample is scanned with an electron beam, and an image in apre-determined observation field on the sample is displayed on an imagedisplay device by using an image signal obtained by detecting secondaryelectrons emitted from the sample. The seventh scanning electronmicroscope has: a sample stage for two-dimensionally moving the sampleon the electron beam scanning plane; a displacement information settingdevice for setting information concerning displacement of the samplestage in first and second directions intersecting each other throughencoders for two axes each of which converts an amount of displacementinto an electric signal; a moving mode setting unit for setting a movingmode of the sample stage to either a position control mode or a speedcontrol mode; and a stage controller for controlling the sample stagesuch that, when the position control mode is set through the moving modesetting unit, the stage controller moves the sample stage to a positioncorre-sponding to the amount of change of displacement information setthrough the displacement information setting device, whereas, when thespeed control mode is set through the moving mode setting unit, thestage controller moves the sample stage at a speed corresponding to theamount of change of displacement information set through thedisplacement information setting device.

In this case, one example of the displacement information setting deviceis a track ball having a rotating member, and two rotary encoders forconverting the rotational angles of different surfaces of the rotatingmember into electric signals.

According to the first to fourth scanning electron microscopes of thepresent invention, the stage for positioning the sample is driventhrough the feed screws by the pulse motors driven by a micro-step drivecontrol method. In the micro-step drive control, the smallest step angleof the pulse motor can be reduced to about 1/100 of that in the case ofthe full-step or half-step drive control. Accordingly, the positioningresolution can be made more than one order in magnitude smaller than thewidth of the observation field usually set in scanning electronmicroscopes.

In the first scanning electron microscope, an amount of backlash in thefeed screws of the stage is previously obtained and stored in thebacklash memory, and when the stage moving direction is reversed duringthe drive of the stage, the amount by which the stage is to be moved iscorrected on the basis of the amount of backlash stored in the memory.At this time, since the stage positioning resolution can be raised to ahigh level by virtue of the micro-step drive control, the amount ofmovement of the stage can be accurately corrected even if the amount ofbacklash is of the order, for example, of 1 μm.

According to the second scanning electron microscope, when theobservation field is electrically rotated on the sample through an anglee with respect to the X-axis, a magnified image of the rotatedobservation field is displayed on the screen of the image displaydevice. Thereafter, if the operator issues an instruction to move theimage toward the right side, the stage moves in a direction tilted atthe angle θ with respect to the X-axis; Consequently, the observationfield gradually moves from one region to another. At this time, sincethe positioning resolution of the stage as moved by the micro-step drivecontrol method can be made more than one order in magnitude finer thanthe width of the observation field usually set in scanning electronmicroscopes, the observation field can be accurately and smoothly moved.

According to the third scanning electron microscope, even when theobservation magnification is changed, the moving speed of the sampleobservation image on the screen of the image display device is keptconstant. Accordingly, the controllability improves. Since the stage canbe moved with high resolution by the micro-step drive control, theobservation image moving speed can also be controlled with highaccuracy.

According to the fourth scanning electron microscope, pitch errors ofthe feed screws of the stage are previously obtained and stored in thepitch error memory, and the amount by which the stage is to be moved iscorrected on the basis of the pitch errors stored in the memory when thestage is to be driven. At this time, since the stage positioningresolution can be raised to a high level by virtue of the micro-stepdrive control, the amount of movement of the stage can be accuratelycorrected even if the pitch errors are of the order, for example, of 1μm.

According to the fifth scanning electron microscope of the presentinvention, when a direction in which the observation field is to bemoved is designated through the field moving direction designating unit,the field movement control unit obtains an actual width of theobservation field on the sample on the basis of the observationmagnification and the observation field on the sample, which have beenset through the magnification and field setting unit. Assuming that theobservation field is a rectangle with a width DX in the direction X andwith a width DY in the direction Y, and that the observation field is tobe moved to a region which is adjacent to the present position in thedirection +X, the field movement control unit moves the sample by awidth DX in the direction -X by operating the sample positioning device.Consequently, the desired region moves into the observation field.

When the observation field is to be moved to a region which is adjacentto the present position in an oblique direction, the sample is obliquelymoved, for example, by a width DX in the direction +X and by a width DYin the direction +Y. Consequently, the region obliquely moves into theobservation field. Accordingly, the observation field can be moved tothe adjacent region at high speed and with high accuracy independentlyof the observation magnification.

In a case where the sample positioning device is a feed screw-operatedstage which is moved by pulse motors driven by a micro-step drivecontrol method, the pulse motors can be rotated and stopped withextremely fine angular resolution by the micro-step drive control.Accordingly, the stage feed resolution can be made considerably smallerthan the width of the observation field of the scanning electronmicroscope, which is very small. Therefore, the observation field can beaccurately moved to a neighboring region.

When the observation field is a region variable in shape, which isselected from the area on the sample within which the sample surface isscanned with the electron beam, the shape and size of the observationfield can be electrically adjusted.

According to the sixth scanning electron microscope of the presentinvention, when the observation field on the sample is desired to movein the first direction (direction X), for example, the moving directionis limited to the first direction through the moving direction limitingunit. Thereafter, even if a direction slightly deviating from the firstdirection, for example, is set when a moving direction is to be setthrough the moving direction input device, the observation field movesin the first direction.

In a case where the moving direction input device also sets a movingspeed, when the moving direction is limited to the first directionthrough the moving direction limiting unit, the observation field on thesample moves in the first direction at a speed component in the firstdirection of the moving speed set through the moving direction inputdevice.

According to the seventh scanning electron microscope of the presentinvention, when the observation field is desired to move to a nearposition on the sample, for example, the position control mode is setthrough the moving mode setting unit. Thereafter, when displacementinformation is generated from a displacement information setting devicehaving a track ball, for example, the sample stage moves to a positioncorresponding to the amount of change of the displacement information,causing the observation field to move as desired. Next, when theobservation field is desired to move to a faraway position on thesample, the speed control mode is set through the moving mode settingunit. Thereafter, when displacement information is generated from thedisplacement information setting device, the sample stage moves at aspeed corresponding to the amount of change of the displacementinformation, causing the observation field to move toward the desiredposition.

When the observation field reaches the vicinity of the desired position,the displacement information generated from the displacement informationsetting device is returned to the previous state. Consequently, thesample stage stops. Alternatively, the moving mode may be changed to theposition control mode when the observation field approaches the vicinityof the desired position. By such an operation, the observation field canbe readily moved to various positions as desired without increasing theamount of control operation required at the displacement informationsetting device.

According to another scanning electron microscope of the presentinvention in which a surface of a sample is scanned with an electronbeam, and an image in a predetermined observation field on the sample isdisplayed on an image display device by using an image signal obtainedby detecting secondary electrons emitted from the sample, the scanningelectron microscope comprises a positioning device for two-dimensionallymoving the sample on a plene which is scanned with the electron beam;and a drive device for moving the sample by a distance substantiallyequal to a width of the observation field in a moving direction of theobservation field by driving the positioning device.

According to the other scanning electron microscope of the presentinvention, the scanning electron microscope comprises a stage fortwo-dimensionally moving a sample of which surface is scanned with anelectron beam, a track ball for setting as desired a moving direction ofthe stage and a device for limiting the moving direction of the stageonly to a predetermined direction. The stage is moved in saidpredetermined direction while said limiting device operates.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram partly containing a perspective view, showingone embodiment of the scanning electron microscope according to thepresent invention.

FIG. 2(a) shows the way in which a pulse motor is driven by a full-stepdrive control method.

FIG. 2(b) shows the way in which a pulse motor is driven by a micro-stepdrive control method.

FIG. 3 illustrates the connection of coils of a pulse motor shown inFIG. 1, and also shows the arrangement of one example of a movementcontroller for micro-step drive control.

FIG. 4 illustrates the connection of coils of a pulse motor shown inFIG. 1, and also shows the arrangement of another example of a movementcontroller for micro-step drive control.

FIG. 5 is a flowchart showing one example of a backlash correctingmethod employed in the embodiment.

FIGS. 6(a) and 6(b) show an observation field on a sample, and amagnified image thereof.

FIGS. 7(a) and 7(b) show a magnified image observed when an observationfield is rotated in the embodiment.

FIGS. 8(a), 8(b), 8(c) and 8(d) illustrate an operation performed when arotated observation field is horizontally moved in the embodiment.

FIGS. 9(a), 9(b), 9(c) and 9(d) illustrate an operation performed when arotated observation field is horizontally moved in a conventionaltechnique.

FIG. 10 shows the way in which an observation field is moved at aconstant speed on a display screen.

FIG. 11 shows one example of pitch error of a feed screw in theembodiment.

FIG. 12 is a block diagram partly containing a perspective view, showingone embodiment of the scanning electron microscope according to thepresent invention.

FIG. 13 shows eight regions which are adjacent to an observation fieldon a sample in the embodiment shown in FIG. 12.

FIGS. 14(a), 14(b) and 14(c) illustrate stepwise movement of anobservation field in the embodiment shown in FIG. 12.

FIG. 15 is a block diagram partly containing a perspective view, showingone embodiment of the scanning electron microscope according to thepresent invention.

FIG. 16 shows the arrangements of a field moving direction limitingunit, a track ball, etc. in the embodiment shown in FIG. 15.

FIGS. 17(a) and 17(b) show an observation field on a sample, and anobservation image thereof.

FIGS. 18(a) and 18(b) show a rotated observation field, and anobservation image thereof.

FIGS. 19(a), 19(b), 19(c) and 19(d) illustrate an operation performedwhen a rotated observation field is horizontally moved.

FIG. 20 illustrates the way in which an observation field is moved at aconstant speed on a display screen.

FIG. 21 is a block diagram partly containing a perspective view, showingone embodiment of the scanning electron microscope according to thepresent invention.

FIG. 22 shows the arrangements of a position mode/speed mode selectingswitch, a track ball, etc. in the embodiment shown in FIG. 21.

FIGS. 23(a) and 23(b) show an observation field on a sample, and anobservation image thereof.

FIGS. 24(a) and 24(b) show a rotated observation field, and anobservation image thereof.

FIGS. 25(a), 25(b), 25(c) and 25(d) illustrate an operation performedwhen a rotated observation field is horizontally moved.

FIG. 26 illustrates the way in which an observation field is moved at aconstant speed on a display screen.

FIG. 27 shows a characteristic curve showing the change in absolutevalue of the moving speed of a sample stage when an observation field ismoved in a position mode.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

One embodiment that incorporates the features of the first to fourthscanning electron microscopes of the present invention will be describedbelow with reference to the accompanying drawings.

FIG. 1 shows a mechanical part and control system of a scanning electronmicroscope according to this embodiment. Referring to FIG. 1, anelectron gun 2, which is disposed in the top of the scanning electronmicroscope, emits an electron beam 3. The electron beam 3 passes throughan aperture 4 and is then converged by a converging lens 5. Thereafter,the electron beam 3 is deflected in directions X and Y by X- andY-deflecting coils 6 and 7, respectively. It should be noted that twoorthogonal directions on a plane perpendicular to the optical axis ofthe electro-optical system are herein defined as directions X and Y,respectively.

The electron beam 3, which has been deflected in the two directions, isfurther passed through an objective lens 8 so as to converge on thesurface of a sample 9 in a sample chamber (not shown), thereby scanninga predetermined region of the sample surface. The sample 9 is placed ona sample holder 11 which, in turn, is mounted on a Y-stage 12Y. TheY-stage 12Y is placed on an X-stage 12X and engaged with a feed screw13Y which extends in the direction Y. The Y-stage 12Y can be driven tomove in the direction Y by rotating the feed screw 13Y in forward andbackward directions with a pulse motor (i.e. stepping motor) 14Y.Further, the X-stage 12X is placed on a base 15 and engaged with a feedscrew 13X which extends in the direction X. The X-stage 12X can bedriven to move in the direction X by rotating the feed screw 13X inforward and backward directions with a pulse motor 14X. In thisembodiment, the pulse motors 14X and 14Y are driven by respectivemovement control units 20X and 20Y for micro-step drive control. Thatis, the pulse motors 14X and 14Y are driven by a microstep drive controlmethod (detailed later).

In addition, a moving mirror 16X having a reflecting surfaceperpendicular to the X-axis is secured to the Y-stage 12Y, and a movingmirror 16Y having a reflecting surface perpendicular to the Y-axis isalso secured to the Y-stage 12Y. Thus, the Y-coordinate of the sampleholder 11 is measured by a combination of a laser interferometer 17X andthe moving mirror 16X, and the X-coordinate of the sample holder 11 ismeasured by a combination of a laser interferometer 17Y and the movingmirror 16Y. The coordinates (X, Y) measured by the laser interferometers17X and 17Y are supplied to a stage controller 19 in a central controlsystem 18 which generally controls the operation of the entireapparatus.

It should be noted that, in this embodiment, the stages 12X and 12Y aredriven at a moving step close to the measuring resolution of the laserinterferometers by a micro-step drive control method; therefore, it isnot always necessary to provide the laser interferometers 17X and 17Y.In this embodiment, therefore, measured values obtained by the laserinterferometers 17X and 17Y are mainly used for monitoring to checkwhether or not the moving speed or position of the stage 12X or 12Y hasdeviated from the specified speed or position because of the step-out ofthe pulse motor 14X or 14Y.

Further, a secondary electron detector 22 is disposed obliquely abovethe sample 9 to detect secondary electrons 21 emitted from the surfaceof the sample 9 as a result of irradiation with the electron beam 3. Adetected signal S1 from the secondary electron detector 22 is passedthrough an amplifier 23 and an analog-to-digital converter (not shown)and then stored as image data in a frame memory provided in an imageprocessing circuit 24. The image processing circuit 24 is also suppliedwith information indicating an observation field, together with asynchronizing signal corresponding to the electron beam scanning, froman electro-optical system control circuit 28. The image processingcircuit 24 reads out image data corresponding to the observation fieldfrom the frame memory, and supplies it, together with the scanningsynchronizing signal, to a CRT display 25. The display screen of the CRTdisplay 25 displays an enlarged image of an object in the observationfield on the sample 9 at a predetermined magnification. The displayscreen of the CRT display 25 also displays information for designating amagnification, a moving direction of the observation field, etc.

To use the scanning electron microscope, the operator needs to set amagnification, an observation field, etc. Therefore, the following unitsare connected to the central control system 18 in this embodiment: amagnification and field setting unit 26 for setting a magnification, arotational angle of observation field, and an observation field; a stagespeed setting unit 29 for setting a moving speed and moving direction ofthe stages 12X and 12Y; a stage position setting unit 30 for setting amoving direction and moving distance of the stages 12X and 12Y; and adata input unit 31 for inputting an amount of backlash of the stages 12Xand 12Y and other data, as described later.

More specifically, the magnification and field setting unit 26 comprisesa keyboard and a mouse. The operator sets the numerical value of anobservation magnification displayed on the CRT display 25 by keying init from the keyboard, and also sets a rotational angle of observationfield by a key-in operation. Further, the operator cuts a desiredportion from the magnified sample image displayed on the display screenof the CRT display 25 by a mouse operation to thereby set an observationfield. The signals from the keyboard and the mouse are supplied to amagnification setting storage device 27 in the central control system18. The magnification setting storage device 27 stores informationconcerning the designated magnification and rotational angle, togetherwith positional information concerning the contour of the setobservation field, in its internal memory, and supplies the informationconcerning the magnification, rotational angle and observation field tothe electro-optical system control circuit 28.

The electro-optical system control circuit 28 outputs control signalscorresponding to the designated magnification and observation fieldrotational angle to an electro-optical system which comprises the X- andY-deflecting coils 6 and 7, etc. The electro-optical system controlcircuit 28 also outputs a synchronizing signal synchronized with acontrol signal for scanning of the electron beam by the X- andY-deflecting coils 6 and 7, together with the observation fieldinformation, to the image processing circuit 24. The electron beam scansacross a scanning range, which corresponds to the magnification, on thesample 9, and rotates the electron beam scanning direction through thedesignated rotational angle. Consequently, a sample image is displayedas a magnified image on the CRT display 25 at the magnificationdesignated by the operator and in the state of being rotated through thedesignated rotational angle.

The image that is first displayed on the display screen of the CRTdisplay 25 is, for example, a magnified image of substantially theentire scan region on the sample 9 which is scanned with the electronbeam, and a magnified image of a partial region selected from the scanregion is cut by a mouse operation. In this embodiment, a region on thesample 9 which corresponds to the magnified image displayed on thedisplay screen of the CRT display 25 is referred to as "observationfield". Accordingly, when a predetermined portion is cut from themagnified image of the entire scan region, a variable partial region onthe sample 9 which corresponds to the cut portion is the observationfield. Cutting of an image is effected by reading out image data for arange which corresponds to the designated observation field from theframe memory in the image processing circuit 24.

The stage speed setting unit 29 comprises a combination of a joy stickand a keyboard. When the operator controls the joy stick serving as thestage speed setting unit 29, signals indicating a moving direction andmoving speed of the stages 12X and 12Y are supplied to the stagecontroller 19. The stage controller 19 drives the stages 12X and 12Y atthe designated speed through the movement control units 20X and 20Y.Such constant-speed movement of the sample stage (12X and 12Y) iscarried out, for example, when the operator desires to move theobservation field in the desired direction while observing a magnifiedsample image displayed on the CRT display 25. The operator can alsodirectly inputs a stage moving direction and moving speed through thekeyboard serving as the stage speed setting unit 29. It should be notedthat an input device, for example, a track ball, may be used in place ofthe joy stick.

Meanwhile, the stage position setting unit 30 comprises a mouse and akeyboard. For example, the operator designates a point (move point) awayfrom the magnified image presently displayed on the CRT display 25 by amouse operation. Information concerning the move point, together withinformation concerning the present observation magnification andobservation field from the magnification setting storage device 27, issupplied to the stage controller 19. The stage controller 19 calculatesa direction and distance from the center of the present observationfield to the move point. Then, the stage controller 19 moves the stages12X and 12Y by the calculated distance in the calculated directionthrough the movement control units 20X and 20Y. Thus, the center of theobservation field moves to the position designated by the operator.

Further, the apparatus in this embodiment is arranged such that, whenthe operator inputs a command to move the stage by 10 mm in thedirection X, for example, from the keyboard serving as the stageposition setting unit 30, the stage controller 19 rotates the pulsemotor 14X through a rotational angle corresponding to 10 mm through themovement control unit 20X in response to the input command.

Next, the data input unit 31 comprises a keyboard. After the amount ofbacklash in each of the stages 12X and 12Y and the pitch error in eachof the feed screws 13X and 13Y have been measured, the operator inputsthe measured backlash and pitch error to a parameter setting device 32in the central control system 18 through the data input unit 31. Inresponse to the input operation, the parameter setting device 32 storesthe amount of backlash and the pitch error in a backlash storage region33a and a pitch error storage region 33d, respectively, in a memory 33.Thereafter, the stage controller 19 makes correction on the basis of theamount of backlash and pitch error read out from the memory 33 when thestages 12X and 12Y are to be driven. In addition, the stage controller19 writes information concerning the advance direction, observationmagnification, rotational angle of observation field, etc., so far set,to the memory 33 through the parameter setting device 32.

It should be noted that the magnification setting storage device 27, thestage controller 19 and the parameter setting device 32, which areprovided in the central control system 18 in this embodiment, arefunctions which are executed by software of a computer.

Next, the micro-step drive control of the pulse motors 14X and 14Y inthis embodiment will be explained in detail.

In general, pulse motors (stepping motors) rotate and stop for each stepangle which is determined by a salient-pole structure of a rotor and astator, and hence make it possible to effect positioning control withhigh accuracy and with ease in comparison to DC motors or AC motors. Onthe other hand, pulse motors have such characteristics that, since theyrotate for each step angle, the rotor speed varies, causing resonance tooccur or vibration to increase at a certain number of revolutions.

In this regard, micro-step drive control is a technique in which thebasic step angle of a pulse motor is subdivided by controlling thecurrent passed through the motor coils, thereby realizing ultra-lowspeed drive, low-vibration drive, and low-noise operation, and alsoimproving the positioning resolution, and thus enabling the positioningaccuracy to be further improved.

More specifically, FIG. 2(a) shows a two-phase pulse motor having abasic step angle of 90°. Referring to FIG. 2(a), a stator 51 is formedfrom a magnetic material having four projections provided at intervalsof 90°. A rotor 53, which is a magnet formed with opposite magneticpoles, is rotatably disposed in the stator 51 so as to face theprojections. One projection 51a of the stator 51 is wound with a coil52A, and a projection 51b which is adjacent to the projection 51a iswound with a coil 52B.

When the pulse motor shown in FIG. 2(a) is to be driven by full-stepdrive control, the current IA which is to be passed through one coil 52Ais set to a positive maximum value I₀, and the current IB which is to bepassed through the other coil 52B is set to 0. Consequently, the rotor53 comes to rest at a position where it faces the projection 51a, asshown by the solid line. Next, the current IA is set to 0, and the othercurrent IB is set to a positive maximum value I₀. Consequently, therotor 53 rotates through 90° and comes to rest at a position where itfaces the projection 51b, as shown by the dotted line. By alternatelyexciting the currents IA and IB in this way, the rotor 53 is caused torotate with a basic step angle of 90°.

Next, when the pulse motor shown in FIG. 2(a) is to be driven bymicro-step drive control, the ratio of the current IA to the current IBis gradually changed. For example, when the current IA is (1/4)I₀ andthe current IB is (3/4)I₀, as shown in FIG. 2(b), the. rotor 53 comes torest after rotating through approximately (3/4)×90° from the projection51a toward the projection 51b. Accordingly, the smallest step angle canbe limitlessly reduced in principle by finely setting the ratio of thecurrent IA to the current IB. This is micro-step drive control.

In this embodiment, a five-phase pulse motor having a basic step angle φof 0.72° is used as one example of the pulse motors 14X and 14Y.Further, the basic step angle is subdivided into about 100 step anglesby the micro-step drive control. An example of the arrangement of themovement control units 20X and 20Y for the micro-step drive controlwhich is used in this embodiment will be explained below.

FIG. 3 shows five-phase coils 52A to 52E of the pulse motor 14X for theX-axis, and the movement control unit 20X for controlling the current tobe supplied through each of these coils. Referring to FIG. 3, the coils52A to 52E are connected with current control circuits 54A to 54E havingthe same arrangement for supplying currents thereto independently ofeach other.

For example, in the current control circuit 54A for the A-phase, theemitters of pnp-type transistors 56A and 58A are connected to a powersupply terminal to which a DC voltage V_(cc) is applied, and the coil52A is connected between the collectors of the transistors 56A and 58A.In addition, the collector of an npn-type transistor 57A is connected tothe collector of the transistor 56A, and the collector of an npn-typetransistor 59A is connected to the collector of the transistor 58A. Theemitters of the transistors 57A and 59A are connected in common to oneend of a resistor 60A. The other end of the resistor 60A is grounded.The bases of the transistors 56A, 57A, 58A and 59A and the one end ofthe resistor 60A are connected to an A-phase control switching circuit55A. The A-phase control switching circuit 55A controls the currentflowing through each of the transistors 56A, 57A, 58A and 59A.

The current control circuits 54B to 54E for the other coils 52B to 52Ealso include respective control switching circuits (not shown) havingthe same arrangement as that of the A-phase control switching circuit55A.

A control signal SX from the stage controller 19, shown in FIG. 1, issupplied to a phase-by-phase switching circuit 61 shown in FIG. 3. Thephase-by-phase switching circuit 61 controls the switching and currentcontrol operation of each control switching circuit (55A, etc.) in thecurrent control circuits 54A to 54E so that the pulse motor 14X rotatesthrough a rotational angle corresponding to a distance designated by thecontrol signal SX. Consequently, the pulse motor 14X rotates in thedesignated direction through the rotational angle designated in units ofthe smallest step angle of 0.72°/100. The movement control unit 20Y forthe Y-axis pulse motor 14Y also has the same arrangement as that of themovement control unit 20X for the X-axis.

It should be noted that the arrangement of the movement control unitsfor the pulse motors 14X and 14Y is not necessarily limited to theexample shown in FIG. 3, and that the movement control units may bearranged as shown in FIG. 4.

FIG. 4 shows five-phase coils 52A to 52E of the pulse motor 14X and alsoillustrates the arrangement of another example of the movement controlunit 20X for controlling the current to be passed through each of thesecoils. Referring to FIG. 4, the coils 52A to 52E are serially connectedso as to form a closed loop. The emitters of pnp-type transistors 56A to56E are connected in common to a power supply terminal to which a DCvoltage V_(cc) is applied. One end of each of the coils 52A to 52E isconnected to the collector of the corresponding transistor (56A to 56E).In addition, the collectors of npn-type transistors 57A to 57E areconnected to the respective collectors of the transistors 56A to 56E.The emitters of the transistors 57A to 57E are connected in common toone end of a resistor 62. The other end of the resistor 62 is grounded.The bases of the transistors 56A to 56E and 57A to 57E and the one endof the resistor 62 are connected to a control switching circuit 63. Thecontrol switching circuit 63 controls the current flowing through eachof the transistors 56A to 56E and 57A to 57E.

In this case, a control signal SX from the stage controller 19, shown inFIG. 1, is supplied to the control switching circuit 63, shown in FIG.4. The control switching circuit 63 controls the current flowing througheach of the coils 52A to 52E so that the pulse motor 14X rotates througha rotational angle corresponding to the distance designated by thecontrol signal SX. Consequently, the pulse motor 14X rotates in thedesignated direction through the rotational angle designated in units ofthe smallest step angle of 0.72°/100.

Referring to FIG. 1, the feed screws 13X and 13Y in this embodiment areassumed to be ball screws having a lead of 5 mm (i.e. 5 mm pitch). Asthe pulse motors 14X and 14Y, five-phase pulse motors having a basicstep angle φ of 0.72° are used, as described above. The pulse motors 14Xand 14Y are driven by the micro-step drive control method with the basicstep angle φ divided into 100 step angles. Accordingly, the feedresolution p of the stages 12X and 12Y is 0.1 μm as given by

    p=5/(100.360/0.72)=1×10.sup.-4  mm!                  (1)

Assuming that the sample observation magnification is 100,000, the widthof the observation field on the sample 9 is determined to be 1.8 μm, forexample, from the relationship between the electron beam scanning regionand the observation magnification. Thus, since the feed resolution p ofthe stages 12X and 12Y is 0.1 μm, the observation field can be smoothlymoved.

In contrast, if the pulse motors 14X and 14Y are driven on the basis ofhalf-step drive control by 4-5 phase excitation as in the conventionalpractice, the smallest step angle is 0.36°, and therefore, the feedresolution of the stages 12X and 12Y is 5 μm. Accordingly, theobservation field having a width of 1.8 μm cannot smoothly be moved bythe conventional method.

Next, an operation of correcting the backlash in the stages 12X and 12Yin this embodiment will be explained with reference to the flowchart ofFIG. 5. First, at step 101 in FIG. 5, a reference wafer is placed on thesample holder 11 in FIG. 1. The reference wafer is prepared to.confirmthe amount of movement of the stage. The reference wafer has scales(marks) previously formed thereon at predetermined intervals so as toserve as references for position in the directions X and Y.

For example, to measure a backlash quantity in the direction X, at step102, the operator issues a command from the stage position setting unit30 to the stage controller 19 so as to move the X-stage 12X in thedirection +X by 10 mm (more precisely, 10.0000 mm; the same shall applyhereinafter) from the present position. In response to the command, thepulse motor 14X rotates through an angle corresponding to 10 mm underthe micro-step drive control. Thereafter, at step 103, the operatorstores in memory an appropriate scale image position on the referencewafer which is being displayed on the CRT display 25. In this case, apredetermined index mark is displayed on the display screen of the CRTdisplay 25, and the distance from the index mark to the selected scaleimage, for example, is recorded.

Next, at step 104, the operator moves the X-stage 12X by 10 mm again inthe direction +X by open loop control through the stage position settingunit 30. Thereafter, at step 105, the operator moves the X-stage 12X by10 mm in the direction -X by open loop control through the stageposition setting unit 30. Subsequently, at step 106, the operatormeasures the amount of displacement between the scale image position ofthe reference wafer previously stored and the present scale imageposition on the display screen of the CRT display 25. At this time, theamount of deviation of the scale image position from the index mark inthe direction +X, obtained at step 103, is subtracted from the amount ofdeviation of the present scale image position from the index mark in thedirection +X, and the length (actual length on the display screen)obtained as a result of the subtraction is divided by the observationmagnification. The value obtained by the division is defined as abacklash quantity BX(-) in the direction -X.

Since the observation magnification is 100,000, for example, thebacklash quantity BX(-) is accurately obtained on the order of 0.1 μm,for example. The operator stores the backlash quantity BX(-) in thebacklash storage region 33a, which is provided in the memory 33, throughthe data input unit 31 and the parameter setting device 32. Similarly, abacklash quantity BX(+) which is observed when the moving direction ofthe X-stage 12X is turned from the direction -X to the direction +X ismeasured, and the measured backlash quantity BX(+) is stored in thebacklash storage region 33a. Further, a backlash quantity BY(-) which isobserved when the Y-stage 12Y is turned to the direction -Y is stored inthe backlash storage region 33a, and a backlash quantity BY(+) observedwhen the Y-stage 12Y is turned to the direction +Y is also stored in thebacklash storage region 33a.

Next, the sample stage is controlled by using the obtained backlashquantities as follows: First, at step 107 in FIG. 5, the operator sets amoving direction (direction ±X) and moving distance ΔX of the X-stage12X, for example, to the stage controller 19 from the stage positionsetting unit 30. At this time, information indicating the movingdirection of the X-stage 12X immediately before the present time haspreviously been stored in the backlash storage region 33a by the stagecontroller 19. Thereafter, at step 108, the stage controller 19 checkswhether or not the designated moving direction of the X-stage 12X haschanged from the direction +X to the direction -X. If Yes is the answerat step 108, the pulse motor 14X is rotated, at step 109, by open loopcontrol through an angle corresponding to a moving distance (ΔX+BX(-))obtained by adding the stored backlash quantity BX(-) to the suppliedmoving distance ΔX. Consequently, the backlash is effectively corrected,and thus the X-stage 12X accurately moves by ΔX in the direction -X.

If the moving direction changes from the direction -X to the direction+X at step 108, the control operation shifts to step 110, at which themoving distance ΔX is corrected on the basis of the backlash quantityBX(+). When there is no change in the moving direction, the movingdistance ΔX is used as it is. Assuming that the backlash quantity is 1μm, for example, the backlash can be accurately corrected in thisembodiment because the positioning resolution of the X-stage 12X israised to the order of 0.1 μm by driving the pulse motor 14X by themicro-step drive control method.

In contrast to the above, it has been difficult with the conventionalsample stage to correct the backlash even if the backlash quantity ismeasured. For example, even if the sample stage is equipped with ballscrews having a lead of 5 mm, and each ball screw is driven with afive-phase pulse motor by 4-5 phase excitation under half-step drivecontrol, the positioning resolution is 5 μm per step. Accordingly, ifthe backlash quantity is of the order of 1 μm, it cannot be corrected bythe half-step drive control method.

Next, one example of an operation performed when the observation fieldon the sample 9 is to be rotated in this embodiment will be explainedwith reference to FIGS. 6(a) to 9(d). First, in FIG. 1, the sample 9 isplaced on the sample holder 11, and an observation image of the sample 9is displayed on the CRT display 25.

FIG. 6(a) shows an observation field 42 set on the sample 9 at thattime. In FIG. 6(a), a rectangular observation field 42 is set overpatterns 41 on the sample 9. FIG. 6(b) shows the display screen 25a ofthe CRT display 25 shown in FIG. 1. A magnified image of a pattern inthe observation field 42 is displayed on the display screen 25a.Accordingly, assuming that the magnification is β, the actual size ofthe observation field 42 in FIG. 6(a) is 1/β of the size of the displayscreen 25a shown in FIG. 6(b).

Next, the operator rotates the observation field 42 on the sample 9through an angle θ (θ herein assumed to be 45°) counterclockwise fromthe X-axis through the magnification and field setting unit 26 shown inFIG. 1. Consequently, the electron beam scanning direction on the sample9 rotates through 45° by the action of the magnification setting storagedevice 27. As a result, a rectangular observation field 43 is set inparallel to a direction which is counterclockwise tilted at 45° withrespect to the direction X over the patterns 41 on the sample 9, asshown in FIG. 7(a). Further, the angle θ is stored in a rotational anglestorage region 33b, which is provided in the memory 33, through thestage controller 19. Consequently, a magnified image of a pattern in theobservation field 43 is displayed on the display screen 25a, as shown inFIG. 7(b). The magnified image is displayed in such a manner that thesides of the rectangular observation field 43 are parallel orperpendicular to the sides of the display screen 25a.

Next, if it is desired to observe a region lying rightward of the rightside of the magnified image shown in FIG. 7(b), the operator issues acommand to move the stages 12X and 12Y, on which the sample 9 is placed,at a predetermined speed in the direction -X through the stage speedsetting unit 29. In response to the command, the stage controller 19reads out the angle θ from the rotational angle storage region 33b ofthe memory 33, and drives the stages 12X and 12Y to move at apredetermined speed in a direction rotated through an angle (180° -θ)clockwise with respect to the direction +X. This is done, for example,by moving the Y-stage 12Y by ΔX.tan θ in the direction -Y as the X-stage12X moves by ΔX in the direction -X.

As a result, as shown in FIG. 8(a), the observation field moves over thepatterns 41 on the sample 9 from a region 43A through a region 43B to aregion 43C . . . along a direction 44 which is counterclockwise tiltedat an angle θ with respect to the direction +X. As the observation fieldsuccessively moves through the regions 43A, 43B and 43C, magnifiedimages such as those shown in FIGS. 8(b), 8(c) and 8(d) are successivelydisplayed on the display screen 25a of the CRT display 25. Accordingly,this embodiment enables the observation field to be moved in a setdirection on the basis of the magnified image displayed on the displayscreen 25a by a mechanical field moving method independently of theamount of rotation of the observation field even when it is electricallyrotated.

In this case, the pulse motors 14X and 14Y are driven by the micro-stepdrive control method, and thus the feed resolution of the stages 12X and12Y is of the order of 0.1 μm, for example. Therefore, even if the widthof the observation field is 1.8 μm, for example, the observation fieldcan be smoothly moved.

Comparison will now be made between the conventional technique and thisembodiment. In the conventional technique, if the operator controls thesystem so as to move the observation field in the direction +X whileobserving the magnified image as shown in FIG. 7(b), for example, theX-stage 12X moves in the direction -X irrespective of the rotationalangle of the observation field. Consequently, the observation fieldmoves in a diagonal direction.

FIG. 9(a) shows the way in which a rotated observation field is moved byusing the conventional technique. In FIG. 9(a), the observation fieldmoves over the patterns 41 on the sample 9 from a region 45A through aregion 45B to a region 45C . . . along the direction +X. As theobservation field successively moves through the regions 45A, 45B and45C, magnified images such as those shown in FIGS. 9(b), 9(c) and 9(d)are displayed on the display screen 25a of the CRT display 25. That is,the observation field undesirably moves in a direction which is notconformable to the operator's intention. This problem arises because theoperation of the stages 12X and 12Y does not follow up informationconcerning the rotation of the observation field on the sample 9 when ithas been electrically rotated.

In the conventional technique, further, even if the sample stage ismoved with the angle corrected by an amount corresponding to therotational angle of the observation field in order to move the sampleobservation image independently of the amount of rotation of theobservation field, it has been difficult to move the image smoothly whenthe observation magnification is high. For example, even if ball screwshaving a lead of 5 mm are used as the feed screws 13X and 13Y, and thepulse motors 14X and 14Y are driven by 4-5 phase excitation underhalf-step drive control, the positioning resolution is 5 μm per step. Onthe other hand, the width of the observation field is 1.8 μm, forexample. Therefore, the observation field cannot smoothly be moved.

The following is a description of one example of an operation performedin this embodiment when the observation field is to be moved after theobservation magnification has been changed. First, the operatordetermines a speed at which the sample observation image is to be movedwithin the effective field range in the display screen of the CRTdisplay 25 through the stage speed setting unit 29. For example, amoving speed is set so that the sample observation image moves acrossthe effective field range in a time period T (T is 3 seconds, forexample, in this embodiment). Assuming that the observation field on thesample 9 is an observation field 42 having a width DX in the directionX, as shown in FIG. 10, and that the observation field 42 is to be movedtoward a region 47 which is adjacent to the present position in thedirection +X in a time period T, the moving speed of the X-stage 12X isset at a speed DX/T in the direction -X. The observation magnificationat this time is assumed to be β₀ (β₀ is 10,000, for example). Theobservation magnification β₀ has been stored in a magnification storageregion 33c, which is provided in the memory 33.

Next, if the operator changes the observation magnification to β (β is20,000, for example) through the magnification and field setting unit26, the observation magnification β after the magnification change isstored into the magnification storage region 33c in the memory 33. Themagnification storage region 33c has also been stored with the previousobservation magnification β₀. Thereafter, if the operator designatesonly a moving direction of the observation field from the keyboardserving as the stage speed setting unit 29, the stage controller 19drives the stages 12X and 12Y to move by the micro-step drive controlmethod so that the moving speed of the image displayed on the displayscreen of the CRT display 25 is equal to the image moving speed at theprevious observation magnification.

More specifically, assuming that the observation field 42 is to be movedin the direction +X in FIG. 10, the stage controller 19 drives theX-stage 12X in the direction -X at a speed V(β) which is determined by

    V(β)=(DX/T)β.sub.0 /β                       (2)

Consequently, the sample observation image moves across the effectivefield range on the display screen of the CRT display 25 in a time periodT. Accordingly, in this embodiment, a magnified image of the samplemoves at a constant speed on the display screen independently of theobservation magnification, and thus the controllability improves.

In the case of the conventional technique, however, once the stagemoving speed is set at the beginning, the stage moving speed ismaintained at the first set value thereafter. Therefore, if the stagemoving speed is first set so that the sample observation image movesacross the effective field range, which is displayed at a magnificationof 10,000 on the display screen, in 3 seconds, for example, when theobservation magnification is changed to 20,000, the time required forthe sample observation image to move across the effective field rangebecomes 1.5 seconds, and when the observation magnification is changedto 100,000, the required time becomes as short as 0.3 second. This isnot convenient for the operator to move the sample observation image.

Further, it has been difficult with the conventional technique to movethe sample observation image at a constant speed. For example, let usassume that the sample observation image is to move across the effectivefield range in 3 seconds, and the observation magnification at that timeis 10,000. In this case, if the feed screws 13X and 13Y are ball screwshaving a lead of 5 mm, and the pulse motors 14X and 14Y are driven by4-5 phase excitation under half-step drive control, the positioningresolution of the stages 12X and 12Y is 5 μm per step. Assuming that thewidth DX of the observation field on the sample 9 at a magnification of10,000 is 18 μm from the relationship between the electron beam scanningwidth and the observation magnification, the number of steps at whichthe sample observation image can move across the effective field rangeis three. Therefore, the stage speed is set so that the sample stagemoves three steps in 3 seconds. Next, if the observation magnificationis set at 100,000, the width of the observation field becomes 1.8 μm. Inthis case, since the positioning resolution is 5 μm per step, when thesample stage is fed by one step, a totally different observation imageis undesirably displayed on the CRT display 25. It will, therefore, beunderstood to be difficult to move the observation field across theeffective field range in 3 seconds when the observation magnification is100,000.

In contrast, if micro-step drive control is used as in this embodiment,the positioning resolution of the stages 12X and 12Y is 0.1 μm.Therefore, the observation field can be moved at a constant speed evenif the observation magnification is 100,000.

Next, one example of an operation performed in this embodiment when thepitch errors in the feed screws 13X and 13Y are to be corrected will beexplained. First, a reference wafer is placed on the sample holder 11 inFIG. 1. The reference wafer has scales formed thereon in advance as areference for moving distance in order to confirm the amount of movementof the sample stage. The scales have previously been calibrated by ahigh-precision laser interferometer or the like. Pitch error quantitiesof the feed screws 13X and 13Y can be measured by comparing the amountsof movement of the stages 12X and 12Y through the pulse motors 14X and14Y with the reference wafer.

For example, let us assume that the X-stage 12X is to be moved by 5.0 mmin the direction +X from a position where the X-coordinate is 0.0 mm byopen loop control. At this time, the system is set so that a referencescale of 0.0 mm comes to the vicinity of the central index mark on thedisplay screen of the CRT display 25 when the X-coordinate is 0.0 mm,and the amount of positional displacement between the index mark and thereference scale is read. Next, the pulse motor 14X is driven by themicro-step drive control method to move the X-stage 12X by 5.0 mm in thedirection +X. Consequently, a reference scale of 5.0 mm of the referencewafer is displayed on the CRT display 25. Therefore, the amount ofdeviation of the reference scale of 0.0 mm, which has previously beenread, is subtracted from the amount of deviation of the reference markof 5.0 mm from the central index mark on the display screen, and theresulting difference is divided by the observation magnification. Thus,it is possible to obtain a pitch error ΔP(X) of the feed screw 13X inthe X-coordinate range of from 0.0 mm to 5.0 mm. This operation iscarried out every 5.0 mm with respect to the direction X, therebyenabling measurement of the pitch error ΔP(X) of the feed screw 13X forthe direction X.

FIG. 11 shows one example of the pitch error AP(X) measured as describedabove. In FIG. 11, the pitch error ΔP(X) changes in the shape of apolygonal line 48 along the coordinate X. Similarly, the pitch errorΔP(Y) of the feed screw 13Y for the direction Y can be measured. Thepitch errors ΔP(X) and ΔP(Y) are stored into the pitch error storageregion 33d in the memory 33, shown in FIG. 1.

The sample stage is controlled by using the obtained pitch errorquantities as follows: First, the operator sets a moving direction andmoving distance of the stages 12X and 12Y to the stage controller 19through the stage position setting unit 30. The stage controller 19calculates amounts by which the X- and Y-stages 12X and 12Y are to bemoved from the set moving direction and moving distance, and thencorrects the calculated amounts of movement by using the pitch errorsΔP(X) and ΔP(Y) stored in the pitch error storage region 33d.

For example, when the X-stage 12X is to be moved from the position X₁ tothe position X₂ in FIG. 11, the stage controller 19 adds the pitch errordifference (ΔP(X₂)-ΔP(X₁)) to the amount of movement (X₂ -X₁) obtainedby calculation. Then, the stage controller 19 drives the pulse motor 14Xby the corrected amount of movement by micro-step drive control and openloop control. Consequently, the X-stage 12X accurately moves by thedistance (X₂ -X₁). Therefore, the observation field can be moved to thedesired position on the sample independently of the adverse effect ofthe pitch error of the feed screw 13X.

In the case of the conventional sample stage, however, even if the pitcherror of the feed screw is measured, it is difficult to correct thepitch error. The reason for this is that the pitch error is generallysmaller than the positioning resolution of the conventional samplestage; therefore, the pitch error cannot be corrected by open loopcontrol.

However, if micro-step drive control is employed as in the case of thisembodiment, even if the pitch error of the feed screws 13X and 13Y is ofthe order of 1 μm, the pitch error can be accurately corrected becausethe positioning resolution of the sample stage is of the order of 0.1μm.

It should be noted that in the above-described embodiment, coordinatevalues (X, Y) measured by the laser interferometers 17X and 17Y are usedto detect step-out of the pulse motors 14X and 14Y. However, a simplerdetector (e.g., an optical linear encoder) may be used for the step-outdetection. If the pulse motor operation is stable, there is no need ofproviding laser interferometers and associated devices.

According to the first to fourth scanning electron microscopes, a sampleis positioned by a feed screw-driven stage which is moved by pulsemotors controlled by the micro-step drive control method. Therefore, thesample positioning resolution can be made more than one order inmagnitude smaller than the ordinary width of the observation field inthe scanning electron microscopes. Accordingly, it is possible to carryout the following operations:

According to the first scanning electron microscope, even if there isbacklash in a stage for positioning a sample, the observation field onthe sample can be accurately moved to a desired position by a mechanicalfield moving method.

According to the second scanning electron microscope, the operator canmove the observation field on the sample, which has been electricallyrotated, toward a region which is adjacent to the present position onthe sample by a mechanical field moving method while viewing the sampleobservation image displayed on the image display device, independentlyof the rotational angle of the observation field. Therefore, positioningof the observation field is easy.

According to the third scanning electron microscope, when theobservation field is to be moved by a mechanical field moving method onthe basis of the sample observation image displayed on the image displaydevice, the sample observation image can be moved at a constant speed onthe image display device independently of the sample observationmagnification no matter how high it is.

Further, according to the fourth scanning electron microscope, even ifthere is a pitch error in a feed screw of a stage for positioning asample, the observation field on the sample can be accurately moved to adesired position by a mechanical field moving method.

Next, one embodiment of the fifth scanning electron microscope accordingto the present invention will be described with reference to FIG. 12.The arrangement shown in FIG. 12 differs from the arrangement shown inFIG. 1 in the following points:

First, the central control system 18 in this embodiment is connectedwith: a magnification and field setting unit 26 for setting amagnification, a rotational angle of observation field, and anobservation field; a stage speed setting unit 29 for setting a movingspeed and moving direction of the stages 12X and 12Y; a stage positionsetting unit 30 for setting a moving direction and moving distance ofthe stages 12X and 12Y; and a field moving signal generator 131 fordesignating a moving direction of the observation field on the sample 9.

It should be noted that a magnification setting storage device 27 and afield moving quantity setting device 119, which are provided in thecentral control system 18, are functions which are executed by softwareof a computer.

The stage speed setting unit 29 is a joy stick. When the operatorcontrols the stage speed setting unit 29, signals indicating a movingdirection and moving speed of the stages 12X and 12Y are supplied to thefield moving quantity setting device 119. The field moving quantitysetting device 119 drives the stages 12X and 12Y at a designated speedthrough movement control units 20X and 20Y. It should be noted that aninput device such as a track ball may be used in place of the joy stick.

Meanwhile, the stage position setting unit 30 comprises a mouse. Forexample, the operator designates a point (move point) away from themagnified image presently displayed on the CRT display 25 by a mouseoperation. Information concerning the move point, together withinformation concerning the present observation magnification andobservation field from the magnification setting storage device 27, issupplied to the field moving quantity setting device 119. The fieldmoving quantity setting device 119 calculates a direction and distancefrom the center of the present observation field to the move point.Then, the field moving quantity setting device 119 moves the stages 12Xand 12Y by the calculated distance in the calculated direction throughthe movement control units 20X and 20Y. Thus, the center of theobservation field moves to the position designated by the operator.

Next, the field moving signal generator 131 comprises a push button anda two-dimensional joy stick (a track ball or the like is also usable).When it is desired to move the observation field to a region adjacent tothe present position (this will hereinafter be referred to as "stepmovement" of the observation field), the operator first pushes the pushbutton once, and then designates a field moving direction with the joystick. A signal from the push button, together with a signal indicatingthe direction from the joy stick, is supplied to the field movingquantity setting device 119. In response to the signals, the fieldmoving quantity setting device 119 supplies control signals SX₁ and SY₁corresponding to the amounts of movement of the stages 12X and 12Y tothe movement control units 20X and 20Y, respectively. In response to thecontrol signals SX₁ and SY₁, the movement control units 20X and 20Ydrive the stages 12X and 12Y by distances required for the step movementthrough the pulse motors 14X and 14Y. Consequently, the center of theobservation field moves to the neighboring region designated by theoperator. The step movement can be continuously performed bycontinuously operating the push button. In a case where the observationfield has been rotated, the step movement takes place to a region whichis adjacent to the rotated observation field.

It should be noted that, since the arrangements of the remainingelements of the scanning electron microscope shown in FIG. 12 are thesame as the arrangements of the corresponding elements shown in FIG. 1,description thereof is omitted. The micro-step drive control of thepulse motors 14X and 14Y in this embodiment is also basically the sameas that explained in the first-described embodiment. Regarding FIGS. 3and 4, which have been used to explain the micro-step drive control, thefield moving quantity setting device 119 in this embodiment supplies thecontrol signal SX₁ to the phase-by-phase switching circuit 61 and alsoto the control switching circuit 63. Since the feed resolution P of thestages 12X and 12Y is 0.1 μm under the same conditions as those in thefirst-described embodiment, the step movement of the observation fieldcan be smoothly effected without any problem.

Next, an operation of the fifth scanning electron microscope for stepmovement of the observation field on the sample 9 will be described indetail.

FIG. 13 shows an observation field 141 set on the sample 9,. In FIG. 13,the observation field 141 is assumed to be a rectangular region (whichmay be a square region) having a width DX₁ in the direction X and awidth DY₁ in the direction Y. It should be noted that FIG. 13 shows acase where the rotational angle θ of the observation field is 0; whenthe rotational angle θ is not 0, one side of the observation field 141is inclined at an angle θ with respect to the X-axis. In FIG. 13, thereare eight regions 142A to as peripheral rectangular regions which areadjacent to the observation field 141, and which have the same size asthat of the observation field 141. In this embodiment, the observationfield 141 can be stepwise moved to any of the eight regions 142A to142H. It should be noted that, when the observation field 141 is to bemoved, for example, to the region 142H, which is adjacent thereto in thedirection +X, the sample 9 is actually moved by the width DX₁ in thedirection -X through the sample stage. Similarly, when the observationfield 141 is to be moved, for example, to the region 142A, which liesobliquely upward of the observation field 141 on the right (as viewed inFIG. 13), the sample 9 is actually moved by the width DX₁ in thedirection -X and by the width DY₁ in the direction -Y through the samplestage.

FIG. 14(a) shows the way in which the observation field 141 is set overcircuit patterns 143 on the sample 9. As shown in FIG. 14(b), magnifiedimages of patterns in the observation field 141, shown in FIG. 14(a),are displayed on the screen 125a of the CRT display 25 (see FIG. 12). Tostepwise move the observation field 141 to the neighboring region 142E,which lies obliquely downward of the observation field 141 on the left,under the illustrated conditions, the operator first pushes the pushbutton in the field moving signal generator 131 once, and then sets themoving direction of the observation field 141 to the left obliquelydownward direction by using the joy stick. Thus, a signal indicating onestep movement, together with a signal indicating the direction of stepmovement, is supplied to the field moving quantity setting device 119 inthe central control system 18.

The field moving quantity setting device 119 calculates the X-directionwidth DX₁ and Y-direction width DY₁ of the observation field 141 set onthe sample 9 on the basis of the information concerning the observationmagnification and observation field from the magnification settingstorage device 27. Then, the field moving quantity setting device 119supplies control signals SX₁ and SY₁ to the movement control units 20Xand 20Y for micro-step drive control so that the movement control units20X and 20Y move the corresponding stages by the width DX₁ in thedirection +X and by the width DY₁ in the direction +Y. In thisembodiment, the feed resolution of the stages corresponding to thesmallest step angle of the pulse motors 14X and 14Y is 0.1 μm. Assumingthat the observation magnification is 100,000, and that the widths DX₁and DY₁ of the observation field 141 are 1.8 μm, for example, each ofthe pulse motors 14X and 14Y should be rotated through an angle which is18 times as large as the smallest step angle. By doing so, the sample 9,in FIG. 14(a), moves by the width DX₁ in the direction +X and by thewidth DY₁ in the direction +Y. Consequently, the observation field 141moves to the region 142E, and magnified images of patterns in the region142E are displayed on the screen 125a of the CRT display 25, as shown inFIG. 14(c).

Further, in this embodiment, if the operator designates a direction byoperating the joy stick while pushing the push button in the fieldmoving signal generator 131, step movement of the observation field iscontinuously carried out in the designated direction.

Further, when the lower side of the observation field 141, as viewed inFIG. 14(a), has been rotated through a rotational angle θ with respectto the X-axis, and the observation field 141 is to be stepped toward theright side thereof, the stages 12X and 12Y are simultaneously driven sothat the sample 9 moves in a direction intersecting the X-axis at theangle θ. By doing so, step movement can be accurately carried out to aneighboring region even when the observation field 141 has been rotated.

Thus, according to this embodiment, the sample positioning resolutioncan be made more than one order in magnitude smaller than the width ofthe observation field of the scanning electron microscope because thepulse motors 14X and 14Y are driven by the micro-step drive controlmethod. Accordingly, the observation field can be accurately stepped toa neighboring region, and therefore, a desired region on the samplesurface can be observed at high speed and with high accuracy.

It should be noted that, although in this embodiment the pulse motors14X and 14Y can be accurately driven by the micro-step drive controlmethod, the drive of the stages 12X and 12Y by the feed screws 13X and13Y involves the problems of backlash and pitch error in the feed screws13X and 13Y. Therefore, it is desirable that backlash quantitiesobserved when the moving direction of each stage is turned, togetherwith pitch errors of the feed screws 13X and 13Y according to position,should be obtained in advance, and that, when the stages 12X and 12Y areto be actually driven, drive control signals should be corrected for thebacklash and the pitch error.

Further, although in the above-described embodiment coordinate values(X, Y) measured by the laser interferometers 17X and 17Y are used todetect step-out of the pulse motors 14X and 14Y, the arrangement may besuch that DC motors, for example, are used in place of the pulse motors14X and 14Y, and the DC motors are driven by closed loop control on thebasis of values measured by the laser interferometers 17X and 17Y. Theclosed loop control also makes it possible to obtain positioningresolution approximately equal to the measurement resolution of thelaser interferometers. Therefore, the step movement of the observationfield can be accurately carried out. However, the closed loop controlrequires a complicated control system, and makes it likely that stagevibration such as hunting will occur. In contrast, in a system whereinpulse motors are driven by micro-step drive control as in the case ofthe above-described embodiment, open loop control may be adopted, andthe control system is therefore simple and stable. Further, if the pulsemotor operation is stable, it becomes unnecessary to provide laserinterferometers, and it is possible to use a simpler detector (e.g. anoptical linear encoder) for the step-out detection.

According to the above-described fifth scanning electron microscope, thesample is moved through the sample positioning device by a distancewhich is equal to the width of the observation field in a directiondesignated through the field moving direction designating device.Accordingly, even if the observation magnification is high, theobservation field can be speedily and accurately moved to a region whichis adjacent to the present position on the sample.

If the sample positioning device is a stage which is driven by a feedscrew system with pulse motors which are controlled by the micro-stepdrive control method, the sample positioning resolution can be made morethan one order in magnitude smaller than the width of the observationfield of the scanning electron microscope. Accordingly, the observationfield can be accurately moved to a neighboring region without effectingfeedback by using a high-resolution measuring device, e.g. a laserinterferometer.

Further, if the observation field is a region variable in shape which isselected as desired from regions on the sample which are scanned with anelectron beam, the observation field can be accurately moved to aneighboring region by the present invention, irrespective of the shapeof the observation field.

Next, one embodiment of the sixth scanning electron microscope accordingto the present invention will be described with reference to FIG. 15. Inthe following description, portions of the arrangement shown in FIG. 15in which the sixth scanning electron microscope differs from thearrangement shown in FIG. 1 will mainly be explained.

The central control system 18 in this embodiment is connected with: amagnification and field setting unit 26 which is used by the operator toset a magnification, a rotational angle of observation field, and anobservation field; a field moving speed setting unit 229 for setting amoving speed (including a moving direction) of the stages 12X and 12Y; afield moving position setting unit 230 for setting a moving directionand moving distance of the stages 12X and 12Y; a field moving directionlimiting unit 234 for limiting the moving direction of the observationfield to a predetermined direction; a stop switch 235 for stopping themovement of the observation field; and a data input unit 31 forinputting backlash quantities of the stages 12X and 12Y and othernecessary data, as will be described later.

The field moving speed setting unit 229 is an input unit having a trackball. When the operator controls the track ball, signals indicatingmoving speeds in the directions X and Y of the observation field on thesample 9 are supplied to the stage controller 19. The field movingdirection limiting unit 234 supplies a signal for limiting the movingdirection of the observation field to a predetermined direction to thestage controller 19. The stage controller 19 drives the stages 12X and12Y through the movement control units 20X and 20Y so that theobservation field is moved on the sample 9 in the designated directionand at the designated speed. Such constant-speed movement of the stages12X and 12Y is carried out, for example, when the operator desires tomove the observation field in the desired direction while observing amagnified image of the sample 9 on the CRT display 25.

Meanwhile, the field moving position setting unit 230 comprises a mouseand a keyboard. For example, the operator designates a point (movepoint) away from the magnified image presently displayed on the CRTdisplay 25 by a mouse operation. Information concerning the move point,together with information concerning the present observationmagnification and observation field from the magnification settingstorage device 27, is supplied to the stage controller 19. The stagecontroller 19 calculates a direction and distance from the center of thepresent observation field to the move point. Then, the stage controller19 moves the stages 12X and 12Y by the calculated distance in thecalculated direction through the movement control units 20X and 20Y.Thus, the center of the observation field moves to the positiondesignated by the operator.

Next, the arrangements of the field moving speed setting unit 229, thefield moving direction limiting unit 234 and the stop switch 235 in thisembodiment will be explained in detail with reference to FIG. 16.

FIG. 16 shows an essential part of an operator console 261 in thisembodiment. Referring to FIG. 16, a rotating member 263 is buried in anopening provided in the surface of the operator console 261. Inside theoperator console 261, a rotating shaft 264X is placed in sliding contactwith the right-hand surface of the rotating member 263, while a rotatingshaft 264Y is placed in sliding contact with the upper surface of therotating member 263, and rotary encoders 265X and 265Y are connected tothe rotating shafts 264X and 264Y, respectively. The rotating member263, the rotating shafts 264X and 264Y, and the rotary encoders 265X and265Y constitute in combination a track ball 262. By rotating therotating member 263 in a rightward direction (or reverse direction),which is indicated by the arrow 270X, an up-down pulse signal indicatinga rotational direction is outputted from the rotary encoder 265X, andthe pulse signal is supplied to a counting pulse input terminal of areversible counter 266X. Similarly, by rotating the rotating member 263in an upward direction (or reverse direction), which is indicated by thearrow 270Y, an up-down pulse signal is outputted from the rotary encoder265Y, and the pulse signal is supplied to a counting pulse inputterminal of a reversible counter 266Y. If the rotating member 263 isrotated in an oblique direction indicated by the arrow 270C, pulsesignals are simultaneously supplied to the two reversible counters 266Xand 266Y.

The reversible counters 266X and 266Y totalize the supplied pulsesignals with plus and minus signs to obtain count values TX and TY,respectively, and supply them to the stage controller 19. The countvalue TX corresponds to the rotational angle of the rotating member 263in the rightward direction (leftward direction in the case of a negativevalue), and the count value TY corresponds to the rotational angle ofthe rotating member 263 in the upward direction (downward direction inthe case of a negative value). The stage controller 19 can recognize thetwodimensional rotational angle of the rotating member 263 of the trackball 262 by the count values TX and TY. Further, the stage controller 19can reset the count values TX and TY to 0 at any time through respectiveclear terminals of the reversible counters 266X and 266Y. The track ball262 and the reversible counters 266X and 266Y constitute in combinationthe field moving speed setting unit 229.

In a case where there is no limitation on the moving direction by thefield moving direction limiting unit 234, described later, and theobservation field remains unrotated, the stage controller 19 drives theX- and Y-stages 12X and 12Y, shown in FIG. 1, through the movementcontrol units 20X and 20Y at respective moving speeds which aredetermined by k(β).TX and k(β).TY using a constant k(β) previouslydetermined in accordance with the observation magnification β, and thecount values TX and TY. Accordingly, the observation field on the sample9 is moved by a mechanical field moving method at a moving speeddetermined according to the rotational direction and angle of therotating member 263 of the track ball 262.

In addition, the stop switch 235 is fixed to the surface of the operatorconsole 261. When the stop switch 235 is actuated, one pulse signal issupplied to the stage controller 19. In response to the pulse signal,the stage controller 19 resets the count values of the reversiblecounters 266X and 266Y to 0. For example, in a case where the operatordesires to stop the observation field on the sample 9 after moving it inthe desired direction at the desired speed by rotating the rotatingmember 263 of the track ball 262, the observation field can be stoppedby returning the rotating member 263 to the previous rotational positionto thereby return the count values TX and TY to 0. However, it isdifficult to completely return the rotating member 263 to the previousrotational position. Therefore, to stop the observation field, theoperator is recommended to remove his/her hand from the rotating member263 of the track ball 262 and to actuate the stop switch 235. By doingso, a pulse signal is supplied to the stage controller 19, and the stagecontroller 19 resets the count values TX and TY of the reversiblecounters 266X and 266Y to 0. Consequently, the moving speed commandvalue becomes 0, and the observation field stops. Thereafter, when therotating member 263 is rotated again, the observation field moves at aspeed corresponding to the angle through which the rotating member 263is rotated from the rotational position at which it has been stopped.

The field moving direction limiting unit 234 is attached to the surfaceof the operator console 261. The field moving direction limiting unit234 comprises a switch 267 which generates one pulse signal each time itis actuated, and a ternary counter 268 which totalizes pulse signalsgenerated from the switch 267 to obtain a 2-bit count value TS. Thecount value TS is supplied to the stage controller 19. When the powersupply is turned on, the count value TS of the counter 268 is 0.Thereafter, each time the switch 267 is actuated, the count value TS ofthe counter 268 cyclically changes in the sequence: 1, 2, 0, and 1 indecimal. The relationship between the count value TS and the movingdirection of the observation field has previously been determined suchthat, when the count value TS is 0, the moving direction of theobservation field is an arbitrary direction on a two-dimensional plane,whereas, when the count value TS is 1 or 2, the moving direction of theobservation field is a direction X or Y. The directions X and Y in thiscase are usually directions which are parallel to the directions X and Yin FIG. 15. However, in a case where the observation field on the sample9 is rotated by changing the electron beam scanning direction, forexample, directions which are defined by rotating the directions X and Yin FIG. 15 through the rotational angle of the observation field arenewly regarded as directions X' and Y', and if the rotating member 263of the track ball 262 is rotated rightwardly, for example, theobservation field can be moved in the direction X'.

Accordingly, in a case where the observation field remains unrotated,when the count value TS is 1 (i.e. the moving direction is limited tothe direction X), the stage controller 19 regards the count value TY ofthe reversible counter 266Y as 0, whereas, when the count value TS is 2(i.e. the moving direction is limited to the direction Y), the countvalue TX of the reversible counter 266X is regarded as 0. When the countvalue TS is 0, the stage controller 19 regards both the two count valuesTX and TY as effective. Under these conditions, the stage controller 19controls the moving speed of the stages 12X and 12Y. Thus, when themoving direction is limited to the direction X, even if the rotatingmember 263 of the track ball 262 is rotated in an arbitrary direction,only the rightward (or leftward) rotational component of the rotationalangle is regarded as effective, and the observation field moves in thedirection X at a moving speed corresponding to the rotational component.

When the moving direction is limited to the direction Y, even if therotating member 263 of the track ball 262 is rotated in an arbitrarydirection, only the upward (or downward) rotational component of therotational angle is regarded as effective, and the observation fieldmoves in the direction Y at a moving speed corresponding to therotational component. When the observation field has been rotated, thecount values TX and TY of the reversible counters 266X and 266Y areassociated with the new directions X' and Y' on the rotated coordinateaxes.

Further, the 2-bit signal indicating the count value TS of the ternarycounter 268 is supplied to a driver circuit for light-emitting devices269X and 269Y which are fixed to the surface of the operator console261. When the count value TS is 1 (i.e. when the moving direction of theobservation field is the direction X), the light-emitting device 269Xturns on, whereas, when the count value TS is 2 (i.e. when the movingdirection of the observation field is the direction Y), thelight-emitting device 269Y turns on. When the moving direction of theobservation field is an arbitrary direction (i.e. a two-dimensionaldirection), both the light-emitting devices 269X and 269Y are off.Accordingly, the operator can recognize the movable direction of theobservation field, which is presently limited through the field movingdirection limiting unit 234, from the lighting conditions of thelight-emitting devices 269X and 269Y.

In this embodiment, the movement control units 20X and 20Y control thepulse motors 14X and 14Y, respectively, by the micro-step drive controlmethod in the same way as in the foregoing embodiments.

It should be noted that the feed resolution p of the stages 12X and 12Yis 0.1 μm under the same conditions as those in the embodiment which hasbeen described with regard to the first to fourth scanning electronmicroscopes. Therefore, the observation field can be smoothly moved.

Further, in this embodiment, DC motors may be used in place of the pulsemotors 14X and 14Y so as to constitute a closed loop together with thelaser interferometers 17X and 17Y, thereby driving the sample holder 11at moving steps close to the measurement resolution of the laserinterferometers.

It should be noted that among the constituent elements shown in FIG. 15,those which have not been described above are similar to thecorresponding constituent elements in FIG. 1. Therefore, descriptionthereof is omitted.

Next, one example of an operation performed in this embodiment when theobservation field set on the sample 9 is to be moved will be explainedwith reference to FIGS. 15 to 20. First, the sample 9 is placed on thesample holder 11 in FIG. 15, and an observation image of the sample 9 isdisplayed on the CRT display 25.

FIG. 17(a) shows an observation field 242 set on the sample 9 at thattime. As shown in FIG. 17(a), a rectangular observation field 242 is setover patterns 241 on the sample 9. FIG. 17(b) shows a display screen225a of the CRT display 25, which is shown in FIG. 15. A magnified imageof a pattern in the observation field 242 is displayed on the displayscreen 225a. Accordingly, assuming that the magnification is β, theactual size of the observation field 242, shown in FIG. 17(a), is 1/β ofthe size of the display screen 225a, shown in FIG. 17(b).

It is assumed that under the above-described conditions, both thelight-emitting devices 269X and 269Y, shown in FIG. 16, are off, andthus the moving direction of the observation field is an arbitrarydirection. When it is desired to move the observation field 242approximately in a right obliquely upward direction indicated by thearrow 248C in FIG. 17(a), the operator rotates the rotating member 263of the track ball 262, shown in FIG. 16, in a right obliquely upwarddirection indicated by the arrow 270C. Consequently, the observationfield 242 moves in the right obliquely upward direction at a speedcorresponding to the rotational angle of the rotating member 263. Whenit is desired to change the speed of movement (the absolute value of themoving speed) of the observation field 242 in that direction, theoperator should control the rotational angle of the rotating member 263.When it is desired to stop the observation field 242 at a desiredposition, the operator should remove his/her hand from the rotatingmember 263 and actuate the stop switch 235.

On the other hand, when it is desired to move the observation field 242precisely in the direction +X as shown by the arrow 248X in FIG. 17(a),the operator first actuates the switch 267 of the field moving directionlimiting unit 234, shown in FIG. 16, to turn on only the light-emittingdevice 269X, thereby limiting the moving direction of the observationfield to the direction X. Once the moving direction has been limited tothe direction X, even if the operator rotates the rotating member 263 ina direction slightly deviating from the rightward direction, as shown bythe arrow 270X', since the stage controller 19 regards only the countvalue TX as effective. Accordingly, the observation field 242 accuratelymoves in the direction +X at a speed approximately corresponding to therotational angle of the rotating member 263. Similarly, when it isdesired to move the observation field 242 precisely in the direction +Y,as shown by the arrow 248Y,. the operator actuates the switch 267, shownin FIG. 16, to turn on only the light-emitting device 269Y, therebylimiting the moving direction to the direction Y, and then rotates therotating member 263 of the track ball 262 approximately upwardly.

Thus, according to this embodiment, the moving direction of theobservation field 242 can be limited with the field moving directionlimiting unit 234 when the operator desires to move the observationfield 242 precisely in the direction X or Y. Therefore, it isunnecessary to rotate the rotating member 263 of the track ball 262precisely in the direction corresponding to the direction X or Y.

Next, an operation performed when the observation field is to beelectrically rotated will be explained. In this case, the operatorrotates the observation field 242 on the sample through an angle θ (θ isherein assumed to be 45°) counterclockwise from the X-axis through theimage magnification and field setting unit 26, shown in FIG. 15. As aresult, the electron beam scanning direction on the sample 9 is rotatedthrough 45° by the action of the magnification setting storage device27, and as shown in FIG. 18(a), a rectangular observation field 243 isset over the patterns 241 on the sample 9 in parallel to a directionwhich is counterclockwise inclined at 45° with respect to the directionX. Further, the angle θ is stored into the rotational angle storageregion 33b in the memory 33 through the stage controller 19, shown inFIG. 15. Thus, a magnified image of a pattern in the observation field243 is displayed on the display screen 225a, as shown in FIG. 18(b). Themagnified image is displayed in such a manner that each side of therectangular observation field 243 is parallel or perpendicular to thesides of the display screen 225a.

Next, when it is desired to observe a region which lies preciselyrightward of the right-hand side of the magnified image displayed on thedisplay screen 225a in FIG. 18(b), the operator first limits the movingdirection to the direction X with the field moving direction limitingunit 234, shown in FIG. 16, and then rotates the rotating member 263 ofthe track ball 262 approximately rightwardly. Consequently, the stagecontroller 19 regards only the count value TX corresponding to themoving speed in the direction X as effective. The stage controller 19reads out the angle θ from the rotational angle storage region 33b inthe memory 33, and causes the stages 12X and 12Y to move in a directionrotated through the angle (180°-θ) clockwise with respect to thedirection +X at a speed corresponding to the count value TX. Thismovement of the stages 12X and 12Y can be realized, for example, bymoving the Y-stage 12Y by ΔX.tan θ in the direction -Y while moving theX-stage 12X by ΔX in the direction -X.

Consequently, as shown in FIG. 19(a), the observation field moves overthe patterns 241 on the sample 9 from a region 243A through a region243B to a region 243C . . . along a direction 244 which iscounterclockwise tilted precisely at an angle θ with respect to thedirection +X. As the observation field successively moves through theregions 243A, 243B and 243C, magnified images such as those shown inFIGS. 19(b), 19(c) and 19(d) are successively displayed on the displayscreen 225a of the CRT display 25. Accordingly, this embodiment enablesthe observation field to be moved in a set direction on the basis of themagnified image displayed on the display screen 225a by a mechanicalfield moving method independently of the amount of rotation of theobservation field even when it is electrically rotated.

The following is a description of one example of an operation performedin this embodiment when the observation field is to be moved after theobservation magnification has been changed. At this time, the operatordetermines a speed at which the sample observation image is to be movedwithin the effective field range in the display screen of the CRTdisplay 25, for example, through the data input unit 231, shown in FIG.15. For example, a moving speed is set so that the sample observationimage will move across the effective field range in a time period T (Tis 3 seconds, for example, in this embodiment). Assuming that theobservation field on the sample 9 is an observation field 242 having awidth DX₂ in the direction X, as shown in FIG. 20, and that theobservation field 242 is to be moved toward a region 247 which isadjacent to the present position in the direction +X in a time period T,the moving speed of the X-stage 12X is set at a speed DX₂ /T in thedirection -X. The observation magnification at this time is assumed tobe β₀ (β₀ is 10,000, for example). The observation magnification β₀ hasbeen stored in a magnification storage region 33c, which is provided inthe memory 33.

The rotational angle of the rotating member 263 of the track ball 262(shown in FIG. 16) which is required to obtain the stage moving speedDX₂ /T has previously been set to a predetermined angle (hereinafterreferred to as "reference rotational angle") through which the rotatingmember 263 can be readily rotated in a single operation by the operator.Accordingly, if the operator rotates the rotating member 263 rightwardlythrough an angle approximately equal to the reference rotational angle,the observation field 242 moves at such a speed that it crosses theeffective field range of the display screen 225a approximately in thetime period T.

Next, if the operator changes the observation magnification to β (β is20,000, for example) through the magnification and field setting unit26, the observation magnification β after the magnification change isstored into the magnification storage region 33c in the memory 33. Themagnification storage region 33c has also been stored with the previousobservation magnification β₀. Thereafter, if the operator rotates therotating member 263 of the track ball 262, shown in FIG. 16, rightwardlythrough an angle approximately equal to the reference rotational angleafter limiting the moving direction to the direction X, the stagecontroller 19 drives the X-stage 12X so that the moving speed of theimage displayed on the display screen 225a of the CRT display 25 isequal to the image moving speed at the previous observationmagnification.

More specifically, assuming that the observation field 242 is to bemoved in the direction +X in FIG. 20, the stage controller 19 drives theX-stage 12X in the direction -X at a speed V(β) which is determined bythe above-described Eq. (2)

Consequently, the sample observation image moves across the effectivefield range on the display screen of the CRT display 25 approximately inthe time period T. Accordingly, in this embodiment, when the rotationalangle of the rotating member 263 of the track ball 262 is the same, themagnified image of the sample moves at a constant speed on the displayscreen independently of the observation magnification, and thus thecontrollability improves.

Although in the above-described embodiment the track ball 262 is used asthe field moving speed setting unit 229, an input device such as a joystick may also be used in place of the track ball 262.

As has been described above, the sixth scanning electron microscope isprovided with the moving direction limiting device, which enables themoving direction of the sample stage to be set in a first direction, asecond direction, or an arbitrary direction. Therefore, the observationfield can be accurately moved, for example, in a direction parallel tothe first direction simply by inputting a moving direction approximatelycoincident with the first direction with the moving direction inputdevice after limiting the moving direction of the sample stage to thefirst direction.

Further, in a case where the moving direction input device sets atwo-dimensional moving speed of the sample stage in the first and seconddirections, and the stage controller drives the sample stage at thespeed component in the direction limited by the moving directionlimiting device within the moving speed set through the moving directioninput device, the observation field can be accurately moved at thedesignated moving speed in the direction limited through the movingdirection limiting device simply by approximately inputting a movingdirection and a moving speed with the moving direction input device.

When the moving direction input device is a track ball, it isparticularly difficult to rotate the rotating member of the track ballprecisely in a direction corresponding to a predetermined movingdirection. Therefore, by using such a moving direction input device incombination with the moving direction limiting device as in the presentinvention, it is possible to satisfy both the demanded controllabilityand accuracy.

Next, one embodiment of the seventh scanning electron microscopeaccording to the present invention will be described with reference toFIG. 21. In the following description, portions of the arrangement shownin FIG. 21 in which the seventh scanning electron microscope differsfrom the arrangement shown in FIG. 1 will mainly be explained.

The central control system 18 in this embodiment is connected with: amagnification and field setting unit 326 which is used by the operatorto set a magnification, a rotational angle of observation field, and anobservation field; a field movement information setting unit 329 forsetting a moving speed of the stages 12X and 12Y, or informationconcerning the move position (movement information); a positionmode/speed mode selecting switch 336 which is used by the operator toselect either a position mode or a speed mode and designate the selectedmode to the stage controller 19; a field moving direction limiting unit334 for limiting the moving direction of the observation field to apredetermined direction; a stop switch 335 for stopping the movement ofthe observation field; and a data input unit 31 for inputting backlashquantities in the stages 12X and 12Y.

The field movement information setting unit 329 is an input devicehaving a track ball. When the operator controls the track ball, movementinformation concerning the observation field on the sample 9 is suppliedto the stage controller 19. When the position mode is selected throughthe position mode/speed mode selecting switch 336, the stage controller19 regards the movement information as information concerning thedirection and distance to a position to which the observation fieldshould move, whereas, when the speed mode is selected through theposition mode/speed mode selecting switch 336, the stage controller 19regards the movement information as information concerning the movingspeed (including the moving direction) of the observation field.

When the speed mode is selected, the field moving direction limitingunit 334 supplies the stage controller 19 with a signal for limiting themoving direction of the observation field to a predetermined direction.The stage controller 19 drives the stages 12X and 12Y through themovement control units 20X and 20Y to move the observation field on thesample 9 in the designated direction at the set speed. Such a stagemovement is effected, for example, in a case where the operator desiresto move the observation field in the desired direction when observing amagnified image of the sample displayed on the CRT display 25.

Further, in this embodiment, the operator can designate a position towhich the observation field is desired to move through a keyboard (notshown). For example, if the operator inputs a command from the keyboardso that the stages 12X and 12Y will move by 10 mm in the direction X,the stage controller 19 rotates the pulse motor 14X through a rotationalangle corresponding to 10 mm through the movement control unit 20X inresponse to the input command.

Next, the arrangements of the field movement information setting unit329, the field moving direction limiting unit 334, the stop switch 335and the position mode/speed mode selecting switch 336 in this embodimentwill be explained in detail with reference to FIG. 22.

FIG. 22 shows an essential part of an operator console 361 in thisembodiment. Referring to FIG. 22, the position mode/speed mode selectingswitch 336 is fixed to the surface of the operator console 361. Adiscrimination signal D1 is supplied from the switch 336 to the stagecontroller 19. The discrimination signal D1 is at a high level "1" whenthe operator keeps his/her hand off the switch 336. When the operatorpushes the switch 336, the discrimination signal D1 changes to a lowlevel "0". When the discrimination signal D1 is at the high level "1",the stage controller 19 recognizes that the position mode has beenselected, whereas, when the discrimination signal D1 is at the low level"0", the stage controller 19 recognizes that the speed mode has beenselected. Further, when the discrimination signal D1 is at the highlevel "1", an indicating element 336a for indicating the position mode,which is annexed to the position mode/speed mode selecting switch 336,turns on. When the discrimination signal D1 is at the low level "0", anindicating element 336b for indicating the speed mode, which is alsoannexed to the switch 336, turns on. Thus, the operator can confirm themode that is presently selected.

A rotating member 363 is buried in an opening provided in the surface ofthe operator console 361. Inside the operator console 361, a rotatingshaft 364X is placed in sliding contact with the right-hand surface ofthe rotating member 363, while a rotating shaft 364Y is placed insliding contact with the upper surface of the rotating member 363, androtary encoders 365X and 365Y are connected to the rotating shafts 364Xand 364Y, respectively. The rotating member 363, the rotating shafts364X and 364Y, and the rotary encoders 365X and 365Y constitute incombination a track ball 362. By rotating the rotating member 363 in arightward direction (or reverse direction), which is indicated by thearrow 370X, an up-down pulse signal indicating a rotational direction isoutputted from the rotary encoder 365X, and the pulse signal is suppliedto a counting pulse input terminal of a reversible counter 366X.Similarly, by rotating the rotating member 363 in an upward direction(or reverse direction), which is indicated by the arrow 370Y, an up-downpulse signal is outputted from the rotary encoder 365Y, and the pulsesignal is supplied to a counting pulse input terminal of a reversiblecounter 366Y. If the rotating member 363 is rotated in an obliquedirection indicated by the arrow 370C, pulse signals are simultaneouslysupplied to the two reversible counters 366X and 366Y.

The reversible counters 366X and 366Y totalize the supplied pulsesignals with plus and minus signs to obtain count values TX and TY,respectively, and supply them to the stage controller 19. The countvalue TX corresponds to the rotational angle of the rotating member 363in the rightward direction (leftward direction in the case of a negativevalue), and the count value TY corresponds to the rotational angle ofthe rotating member 363 in the upward direction (downward direction inthe case of a negative value). The stage controller 19 can reset thecount values TX and TY to 0 at any time through respective clearterminals of the reversible counters 366X and 366Y. The track ball 362and the reversible counters 366X and 366Y constitute in combination thefield movement information setting unit 329.

In this embodiment, assuming that the position mode has been set, andthat there is no limitation on the moving direction by the field movingdirection limiting unit 334 (described later), and the observation fieldremains unrotated, the stage controller 19 regards the supplied countvalues TX and TY as information indicating amounts of displacement inthe directions X and Y to a position where the observation field shouldbe set. More specifically, the stage controller 19 drives the X- andY-stages 12X and 12Y, shown in FIG. 21, through the movement controlunits 20X and 20Y by distances which are determined by q(β)-TX andq(β).TY using a constant q(β), which has previously been determined inaccordance with the observation magnification β, and the count values TXand TY. Accordingly, the observation field on the sample 9 is moved bythe mechanical field moving method by a distance corresponding to therotational angle of the rotating member 363 of the track ball 362 withrespect to the rotational direction of the rotating member 363.

FIG. 27 shows a characteristic curve showing the change in absolutevalue of the moving speed V_(ST) of the stages 12X and 12Y when theobservation field is moved in the position mode as described above. Asshown in FIG. 27, the absolute value of the moving speed V_(ST) changesin the shape of a trapezoid whose upper side is defined by apredetermined constant peak speed V_(P). However, the peak speed V_(P)may be changed so as to be approximately proportional to the distancethrough which the observation field moves. By doing so, the travelingtime becomes approximately constant.

Referring to FIG. 22, when the speed mode has been set, the stagecontroller 19 regards the count values TX and TY as informationindicating speeds of movement of the observation field in the directionsX and Y. In a case where there is no limitation on the moving directionthrough the field moving direction limiting unit 334 (described alter),and the observation field remains unrotated, the stage controller 19drives the X- and Y-stages 12X and 12Y, shown in FIG. 21, through themovement control units 20X and 20Y at moving speeds which are determinedby k(β).TX and k(β).TY using a constant k(β), which has previously beendetermined in accordance with the observation magnification β, and thecount values TX and TY. Accordingly, the observation field on the sample9 is moved by the mechanical field moving method at a moving speedcorresponding to the rotational direction and angle of the rotatingmember 363 of the track ball 362.

Further, the stop switch 335 is fixed to the surface of the operatorconsole 361. When the stop switch 335 is actuated, a pulse signal D2which is at a high level "1" only for a predetermined period of time issupplied to the stage controller 19. In response to the pulse signal D2,the stage controller 19 resets the count values of the reversiblecounters 366X and 336Y to 0. For example, in a case where the operatordesires to stop the observation field after moving it on the sample 9 ina desired direction at a desired speed by rotating the rotating member363 of the track ball 362 when the speed mode has been selected, theobservation field can be stopped by returning the rotating member 363 tothe previous rotational position to thereby reset the count values TXand TY of the reversible counters 366X and 366Y to 0. However, it isdifficult to completely return the rotating member 363 to the previousrotational position. Therefore, to stop the observation field, theoperator is recommended to remove his/her hand from the rotating member363 of the track ball 362 and to actuate the stop switch 335. By doingso, the pulse signal D2, which is supplied to the stage controller 19,changes to the high level "1", and the stage controller 19 resets thecount values TX and TY of the reversible counters 366X and 366Y to 0.Consequently, the moving speed command value becomes 0, and theobservation field stops. Thereafter, when the rotating member 363 isrotated again, the observation field moves at a speed corresponding tothe angle through which the rotating member 363 is rotated from therotational position at which it has been stopped.

The field moving direction limiting unit 334 has a switch 367 and acounter 368. The switch 367 and the counter 368 have the samearrangements and functions as those of the field moving directionlimiting unit 234 in the above-described sixth scanning electronmicroscope.

Accordingly, when the observation field remains unrotated, and the countvalue TS is 1 (i.e. when the moving direction is limited to thedirection X), the stage controller 19 regards the count value TY of thereversible counter 366Y as 0. When the count value TS is 2 (i.e., whenthe moving direction is limited to the direction Y), the stagecontroller 19 regards the count value TX of the reversible counter 366Xas 0. When the count value TS is 0, the stage controller 19 regards boththe two count values TX and TY as effective. Under these conditions, thestage controller 19 controls the moving distance or the moving speed ofthe stages 12X and 12Y. Thus, when the moving direction is limited tothe direction X, even if the rotating member 363 of the track ball 362is rotated in an arbitrary direction, only the rightward (or leftward)rotational component of the rotational angle is regarded as effective,and the observation field moves in the direction X by a distancecorresponding to the rotational component or at a moving speedcorresponding to it.

When the moving direction is limited to the direction Y, even if therotating member 363 of the track ball 362 is rotated in an arbitrarydirection, only the upward (or downward) rotational component of therotational angle is regarded as effective, and the observation fieldmoves in the direction Y by a distance corresponding to the rotationalcomponent or at a moving speed corresponding to it. When the observationfield has been rotated, the count values TX and TY of the reversiblecounters 366X and 366Y are associated with the new directions X' and Y'on the rotated coordinate axes.

The on/off operation of the light-emitting devices 369X and 369Y is alsothe same as that of the light-emitting devices 269X and 269Y in theabove-described sixth scanning electron microscope.

In this embodiment, the movement control units 20X and 20Y control thepulse motors 14X and 14Y, respectively, by the micro-step drive controlmethod in the same way as in the foregoing embodiments.

It should be noted that the feed resolution p of the stages 12X and 12Yis 0.1 μm under the same conditions as those in the embodiment which hasbeen described with regard to the first to fourth scanning electronmicroscopes. Therefore, the observation field can be smoothly moved.

Further, in this embodiment, DC motors may be used in place of the pulsemotors 14X and 14Y so as to constitute a closed loop together with thelaser interferometers 17X and 17Y, thereby driving the sample holder 11at moving steps close to the measurement resolution of the laserinterferometers.

It should be noted that among the constituent elements shown in FIG. 21,those which have not been described above are similar to thecorresponding constituent elements in FIG. 1. Therefore, descriptionthereof is omitted.

Next, one example of an operation performed in this embodiment when theobservation field set on the sample 9 is to be moved will be explainedwith reference to FIGS. 21 to 26. First, the sample 9 is placed on thesample holder 11 in FIG. 21, and an observation image of the sample 9 isdisplayed on the CRT display 25.

FIG. 23(a) shows an observation field 342 set on the sample 9 at thattime. As shown in FIG. 23(a), a rectangular observation field 342 is setover patterns 341 on the sample 9. FIG. 23(b) shows a display screen325a of the CRT display 25, which is shown in FIG. 21. A magnified imageof a pattern in the observation field 342 is displayed on the displayscreen 325a. Accordingly, assuming that the magnification is β, theactual size of the observation field 342, shown in FIG. 23(a), is 1/β ofthe size of the display screen 325a, shown in FIG. 23(b).

Let us assume that under these conditions, the indicating elements 369Xand 369Y, shown in FIG. 22, are off, and the moving direction of theobservation field has been set to an arbitrary direction. First, when itis desired. to move the observation field 342 on the sample 9 to thevicinity of a near position 349C which is obliquely upward of thepresent position, the operator sets the position mode with the positionmode/speed mode selecting switch 336, shown in FIG. 22, placed in areleased state. Thereafter, the operator rotates the rotating member 363of the track ball 362, shown in FIG. 22, in a right obliquely upwarddirection indicated by the arrow 370C. Consequently, the observationfield 342 moves to a right obliquely upward position corresponding tothe rotational angle. If the position is not coincident with theposition 349C shown in FIG. 23(a), the operator should further rotatethe rotating member 363 through an angle corresponding to the amount ofpositional displacement.

When it is desired to move the observation field 342 rapidly to aposition 349X which is away from the present position precisely in thedirection +X in FIG. 23(a), the operator first turns on only theindicating element 369X by actuating the switch 367 of the field movingdirection limiting unit 334, shown in FIG. 22, thereby limiting themoving direction of the observation field to the direction X.Thereafter, even if the operator rotates the rotating member 363 in adirection slightly deviating from the rightward direction, as shown bythe arrow 370X', the stage controller 19 regards only the count value TXas effective. Therefore, the observation field 342 accurately moves to aposition away from the present position by a distance approximatelycorresponding to the rotational angle in the direction +X. Similarly,when it is desired to move the observation field 342 to a position 349Ywhich is away from the present position precisely in the direction +Y,the operator turns on only the indicating element 369Y by actuating theswitch 367, shown in FIG. 22, thereby limiting the moving direction tothe direction Y. Thereafter, the operator rotates the rotating member363 of the track ball 362 approximately upwardly.

Next, when the observation field 342 is to be moved to a position whichis far away from the present position in a right obliquely upwarddirection indicated by the arrow 348C in FIG. 23(a), the operator pushesthe position mode/speed mode selecting switch 336, shown in FIG. 22,with his/her left hand to set the speed mode. Thereafter, while pushingthe switch 336, the operator cancels the limitation on the movingdirection through the field moving direction limiting unit 334. When theposition and speed modes are changed over from one to the other, thestage controller 19 supplies a pulse to the clear terminal of each ofthe reversible counters 366X and 366Y, thereby resetting both the countvalues TX and TY to 0.

Thereafter, the operator rotates the rotating member 363 of the trackball 362, shown in FIG. 22, in a right obliquely upward directionindicated by the arrow 370C with his/her right hand. Consequently, theobservation field 342 moves in the right obliquely upward direction at aspeed corresponding to the rotational angle of the rotating member 363.When it is desired to change the speed of the movement of theobservation field 342 (the absolute value of the moving speed) in thatdirection, the operator controls the rotational angle of the rotatingmember 363. When it is desired to stop the observation field 342 at adesired position, the operator removes his/her hand from the rotatingmember 363, and actuates the stop switch 335.

When it is desired to move the observation field 342 precisely in thedirection +X as shown by the arrow 348X in FIG. 23(a), the operatorfirst turns on only the indicating element 369X by actuating the switch367 of the field moving direction limiting unit 334, shown in FIG. 22,thereby limiting the moving direction of the observation field to thedirection X. Thereafter, even if the operator rotates the rotatingmember 363 in a direction slightly deviating from the rightwarddirection, as shown by the arrow 370X', the stage controller 19 regardsonly the count value TX as effective. Accordingly, the observation field342 accurately moves in the direction +X at a speed approximatelycorresponding to the rotational angle of the rotating member 363.Similarly, when it is desired to move the observation field 342precisely in the direction +Y as shown by the arrow 348Y, the operatorturns on only the indicating element 369Y by actuating the switch 367,shown in FIG. 22, thereby limiting the moving direction to the directionY. Thereafter, the operator rotates the rotating member 363 of the trackball 362 approximately upwardly.

Thus, according to this embodiment, the observation field 342 can bereadily moved to a desired position simply by turning the rotatingmember 363 of the track ball 362 after selecting either the position orspeed mode such that, when the observation field 342 is to be moved to anear position, the position mode is selected, whereas, when theobservation field 342 is to be moved to a faraway position, the speedmode is selected.

Next, an operation performed when the observation field is to beelectrically rotated will be explained. In this case, the operatorrotates the observation field 342 on the sample through an angle θ (θ isherein assumed to be 45°) counterclockwise from the X-axis through theimage magnification and field setting unit 326, shown in FIG. 21. As aresult, the electron beam scanning direction on the sample 9 is rotatedthrough 45° by the action of the magnification setting storage device327, and as shown in FIG. 24(a), a rectangular observation field 343 isset over the patterns 341 on the sample 9 in parallel to a directionwhich is counterclockwise inclined at 45° with respect to the directionX. Further, the angle θ is stored into the rotational angle storageregion 33b in the memory 33 through the stage controller 19, shown inFIG. 21. Thus, a magnified image of a pattern in the observation field243 is displayed on the display screen 325a, as shown in FIG. 24(b). Themagnified image is displayed in such a manner that each side of therectangular observation field 343 is parallel or perpendicular to thesides of the display screen 325a.

Next, when it is desired to observe a region which lies preciselyrightward of the right-hand side of the magnified image displayed on thedisplay screen 325a in FIG. 24(b), the operator pushes the positionmode/speed mode selecting switch 336 to set the speed mode, and in thisstate, limits the moving direction with the field moving directionlimiting unit 334, shown in FIG. 22. Thereafter, he operator rotates therotating member 363 of the track all 362 approximately rightwardly.Consequently, the stage controller 19 regards only the count value TXcorresponding to the moving speed in the direction X as effective. Thestage controller 19 reads out the angle θ from the rotational anglestorage region 33b in the memory 33, and causes the stages 12X and 12Yto move in a direction rotated clockwise through the angle (180°-θ) withrespect to the direction +X at a speed corresponding to the count valueTX. This movement of the stages 12X and 12Y can be realized, forexample, by moving the Y-stage 12Y by ΔX.tan θ in the direction -Y whilemoving the X-stage 12X by ΔX in the direction -X.

Consequently, as shown in FIG. 25(a), the observation field moves overthe patterns 341 on the sample 9 from a region 343A through a region343B to a region 343C . . . along a direction 344 which iscounterclockwise tilted precisely at an angle θ with respect to thedirection +X. As the observation field successively moves through theregions 343A, 343B and 343C, magnified images such as those shown inFIGS. 25(b), 25(c) and 25(d) are successively displayed on the displayscreen 325a of the CRT display 25. Accordingly, this embodiment enablesthe observation field to be moved in a set direction on the basis of themagnified image displayed on the display screen 325a by the mechanicalfield moving method independently of the amount of rotation of theobservation field even when it is electrically rotated.

The following is a description of one example of an operation performedin this embodiment when the observation field is to be moved in thespeed mode after the observation magnification has been changed. At thistime, the operator determines a speed at which the sample observationimage is to be moved within the effective field range in the displayscreen of the CRT display 25, for example, through the data input unit31, shown in FIG. 21. For example, a moving speed is set so that thesample observation image will move across the effective field range in atime period T (T is 3 seconds, for example, in this embodiment).Assuming that the observation field on the sample 9 is an observationfield 342 having a width DX₃ in the direction X, as shown in FIG. 26,and that the observation field 342 is to be moved toward a region 347which is adjacent to the present position in the direction +X in a timeperiod T, the moving speed of the X-stage 12X is set at a speed DX₃ /Tin the direction -X. The observation magnification at this time isassumed to be β₀ (β₀ is 10,000, for example). The observationmagnification β₀ has been stored in a magnification storage region 33c,which is provided in the memory 33.

The rotational angle of the rotating member 363 of the track ball 362(shown in FIG. 22) which is required to obtain the stage moving speedDX₃ /T has previously been set to a predetermined angle (hereinafterreferred to as "reference rotational angle") through which the rotatingmember 363 can be readily rotated in a single operation by the operator.Accordingly, if the operator rotates the rotating member 363 rightwardlythrough an angle approximately equal to the reference rotational angle,the observation field 342 moves at such a speed that it crosses theeffective field range of the display screen 325a approximately in thetime period T.

Next, if the operator changes the observation magnification to β (β is20,000, for example) through the magnification and field setting unit26, the observation magnification β after the magnification change isstored into the magnification storage region 33c in the memory 33. Themagnification storage region 33c has also been stored with the previousobservation magnification β₀. Thereafter, if the operator rotates therotating member 363 of the track ball 362, shown in FIG. 22, rightwardlythrough an angle approximately equal to the reference rotational angleafter limiting the moving direction to the direction X, the stagecontroller 19 drives the X-stage 12X so that the moving speed of theimage displayed on the display screen 325a of the CRT display 25 isequal to the image moving speed at the previous observationmagnification.

More specifically, assuming that the observation field 342 is to bemoved in the direction +X in FIG. 26, the stage controller 19 drives theX-stage 12X in the direction -X at a speed V(β) which is determined bythe above-described Eq. (2)

Consequently, the sample observation image moves across the effectivefield range on the display screen of the CRT display 25 approximately inthe time period T. ccordingly, in this embodiment, when the rotationalangle f the rotating member 363 of the track ball 362 is the ame, themagnified image of the sample moves at a constant speed on the displayscreen independently of the observation magnification, and thus thecontrollability improves. Similarly, the moving speed of the observationfield when it is to be moved in the position mode is also controlled inaccordance with the observation magnification β.

Although in the above-described embodiment the track ball 362 is used asthe field moving speed setting unit 329, an input device such as a joystick may also be used in place of the track ball 362. In thisembodiment, even when the observation field is to be moved to a farawayposition, the required amount of displacement of the encoder can beminimized by using the speed mode. Therefore, it is possible to use evenan input device in which the displacement range of the control shaft islimited as in the case of a joy stick.

According to the seventh scanning electron microscope, when the positioncontrol mode has been set, displacement information set through thedisplacement information setting device is regarded as informationconcerning the moving direction and moving distance, whereas, when thespeed control mode has been set, the displacement information isregarded as information concerning the moving speed (including themoving direction). Accordingly, when the observation field is to bemoved to a near position, the position control mode is selected,whereas, when the observation field is to be moved to a distantposition, the speed control mode is selected, thereby enabling theobservation field to be readily set to a desired position simply bysetting minimal displacement information through the displacementinformation setting device.

When the displacement information setting device is a track ball,particularly, the observation field can be readily set to a desiredposition without increasing the required amount of rotation of therotating member.

The present invention is not necessarily limited to the above-describedembodiments, but may adopt various arrangements without departing fromthe gist of the present invention.

What is claimed is:
 1. A scanning electron microscope in which a surfaceof a sample is scanned with an electron beam, and an image in apredetermined observation field on said sample is displayed on an imagedisplay device by using an image signal obtained by detecting secondaryelectrons emitted from said sample, said scanning electron microscopecomprising:a feed screw-driven stage for two-dimensionally moving saidsample on a plane which is scanned with said electron beam; two pulsemotors for rotationally driving two feed screws, respectively, of saidstage; a micro-step drive controller for driving said two pulse motorsby a micro-step drive control method; a backlash memory for storing anamount of backlash observed when a moving direction of said stage isreversed; and a field movement control unit which, when said observationfield is to be moved by a predetermined amount on said sample, correctssaid predetermined amount of movement on the basis of (1) a movingdirection of said stage immediately prior to the present time; (2) astage moving direction to be taken subsequently; and (3) storagecontents of said backlash memory, and which drives said pulse motorsthrough an angle corresponding to the corrected amount of movementthrough said micro-step drive controller.
 2. A microscope wherein asample placed on a stage is observed through an objective lens, saidmicroscope comprising:a feed screw by which said stage is fed, said feedscrew being provided with said stage; a motor which drives said feedscrew; a controller which controls an amount of driving of said motorand by which said stage is moved with said feed screw; a measuringdevice which measures an amount of backlash of said feed screw when amoving direction of said stage is reversed; and a correcting devicewhich corrects an amount of movement of said stage on the basis of saidamount of backlash measured by said measuring device when a movingdirection of said stage is reversed.
 3. A microscope according to claim2, said measuring device comprising:a movement controller which movessaid stage from a first position to a second position in a predetermineddirection and then moves said stage from the second position to a thirdposition in said predetermined direction by driving said feed screw by apredetermined amount of driving and then turns said stage back in areverse direction with respect to said predetermined direction so thatsaid stage is moved from said third position to a fourth position bydriving said feed screw by the same amount of driving as saidpredetermined amount of driving; a detector which detects a position ofan index pattern of said stage when said stage arrives at said secondposition by the movement of said stage in said predetermined directionand detects a position of said index pattern when said stage arrives atsaid fourth position by the movement of said stage in said reversedirection; and a computing unit which calculates said amount of backlashfrom an amount of deviation between said position of the index patterndetected at said second position and said position of the index patterndetected at said fourth position.
 4. A microscope according to claim 3,wherein an amount of movement of said stage from said first position tosaid second position in the predetermined direction, an amount ofmovement of said stage from said second position to said third positionin the predetermined direction and an amount of movement of said stagefrom said third position to said fourth position in the reversedirection are equal in magnitude.
 5. A microscope wherein a sampleplaced on a stage is observed through an objective lens, said microscopecomprising:a feed screw by which said stage is fed, said feed screwbeing provided with said stage; a motor which drives said feed screw; acontroller which controls an amount of driving of said motor and bywhich said stage is moved with said feed screw; a memory which memorizesan amount of backlash of said feed screw when a moving direction of saidstage is reversed; and a correcting device which corrects an amount ofmovement of said stage on the basis of said amount of backlash memorizedin said memory when a moving direction of said stage is reversed.
 6. Amicroscope according to claim 5, said microscope further comprising:amovement controller which moves said stage from a first position to asecond position in a predetermined direction and then moves said stagefrom the second position to a third position in said predetermineddirection by driving said feed screw by a predetermined amount ofdriving and then turns said stage back in a reverse direction withrespect to said predetermined direction so that said stage is moved fromsaid third position to a fourth position by driving said feed screw bythe same amount of driving as said predetermined amount of driving; adetector which detects a position of an index pattern of said stage whensaid stage arrives at said second position by the movement of said stagein said predetermined direction and detects a position of said indexpattern when said stage arrives at said fourth position by the movementof said stage in said reverse direction; and a computing unit whichcalculates said amount of backlash from an amount of deviation betweensaid position of the index pattern detected at said second position andsaid position of the index pattern detected at said fourth position;said memory memorizing said amount of backlash calculated by saidcomputing unit.
 7. A microscope according to claim 5, wherein saidmicroscope is a scanning electron microscope in which a surface of saidsample is scanned with an electron beam, and an image in a predeterminedobservation field on said sample is displayed on an image display deviceby using an image signal obtained by detecting secondary electronsemitted from said sample.